diff --git a/docs/platforms/creating_platform.rst b/docs/platforms/creating_platform.rst
index 5347be01..9865beec 100644
--- a/docs/platforms/creating_platform.rst
+++ b/docs/platforms/creating_platform.rst
@@ -217,6 +217,8 @@ base template of ``test-builder.py``.
 
         CPPDEFINES=["DEFINE_1", "DEFINE=2", "DEFINE_N"],
 
+        LIBS=["additional", "libs", "here"],
+
         UPLOADER=join("$PIOPACKAGES_DIR", "tool-bar", "uploader"),
         UPLOADCMD="$UPLOADER $SOURCES"
     )
@@ -238,12 +240,10 @@ base template of ``test-builder.py``.
     # The source code of "platformio-build-tool" is here
     # https://github.com/ivankravets/platformio/blob/develop/platformio/builder/tools/platformio.py
 
-    CORELIBS = env.ProcessGeneral()
-
     #
     # Target: Build executable and linkable firmware
     #
-    target_elf = env.BuildFirmware(CORELIBS + ["additional", "libs", "here"])
+    target_elf = env.BuildFirmware()
 
     #
     # Target: Build the .bin file
diff --git a/platformio/builder/scripts/atmelavr.py b/platformio/builder/scripts/atmelavr.py
index c32fe8b1..eacb925b 100644
--- a/platformio/builder/scripts/atmelavr.py
+++ b/platformio/builder/scripts/atmelavr.py
@@ -82,13 +82,11 @@ else:
         UPLOADEEPCMD='"$UPLOADER" $UPLOADERFLAGS -U eeprom:w:$SOURCES:i'
     )
 
-CORELIBS = env.ProcessGeneral()
-
 #
 # Target: Build executable and linkable firmware
 #
 
-target_elf = env.BuildFirmware(["m"] + CORELIBS)
+target_elf = env.BuildFirmware()
 
 #
 # Target: Extract EEPROM data (from EEMEM directive) to .eep file
diff --git a/platformio/builder/scripts/atmelsam.py b/platformio/builder/scripts/atmelsam.py
index 347cb75a..596872c8 100644
--- a/platformio/builder/scripts/atmelsam.py
+++ b/platformio/builder/scripts/atmelsam.py
@@ -68,13 +68,11 @@ env.Append(
     ]
 )
 
-CORELIBS = env.ProcessGeneral()
-
 #
 # Target: Build executable and linkable firmware
 #
 
-target_elf = env.BuildFirmware(["m", "gcc"] + CORELIBS)
+target_elf = env.BuildFirmware()
 
 #
 # Target: Build the .bin file
diff --git a/platformio/builder/scripts/basearm.py b/platformio/builder/scripts/basearm.py
index 0cb81b83..9ae00990 100644
--- a/platformio/builder/scripts/basearm.py
+++ b/platformio/builder/scripts/basearm.py
@@ -50,16 +50,15 @@ env.Replace(
         "-mcpu=${BOARD_OPTIONS['build']['cpu']}"
     ],
 
+    LIBS=["c", "gcc", "m"],
+
     SIZEPRINTCMD='"$SIZETOOL" -B -d $SOURCES'
 )
 
-if env.get("BOARD_OPTIONS", {}).get("build", {}).get("cpu", "")[-2:] == "m4":
+if env.get("BOARD_OPTIONS", {}).get("build", {}).get(
+        "cpu", "")[-2:] == "m4" and env.get("BOARD") != "frdm_k20d50m":
     env.Append(
-        ASFLAGS=[
-            "-mfloat-abi=hard",
-            "-mfpu=fpv4-sp-d16"
-        ],
-        CCFLAGS=[
+        CPPFLAGS=[
             "-mfloat-abi=hard",
             "-mfpu=fpv4-sp-d16",
             "-fsingle-precision-constant"
diff --git a/platformio/builder/scripts/baseavr.py b/platformio/builder/scripts/baseavr.py
index bfe19df8..8a0384f1 100644
--- a/platformio/builder/scripts/baseavr.py
+++ b/platformio/builder/scripts/baseavr.py
@@ -47,6 +47,8 @@ env.Replace(
         "-Wl,--gc-sections"
     ],
 
+    LIBS=["m"],
+
     SIZEPRINTCMD='"$SIZETOOL" --mcu=$BOARD_MCU -C -d $SOURCES'
 )
 
diff --git a/platformio/builder/scripts/frameworks/arduino.py b/platformio/builder/scripts/frameworks/arduino.py
index 77ef6c88..f154d5c9 100644
--- a/platformio/builder/scripts/frameworks/arduino.py
+++ b/platformio/builder/scripts/frameworks/arduino.py
@@ -8,7 +8,7 @@
 from os import listdir, walk
 from os.path import isfile, join
 
-from SCons.Script import DefaultEnvironment, Return
+from SCons.Script import DefaultEnvironment
 
 env = DefaultEnvironment()
 
@@ -178,4 +178,4 @@ if env.subst("${PLATFORMFW_DIR}")[-3:] == "sam":
         join("$PLATFORMFW_DIR", "system", "libsam", "source")
     ))
 
-Return("libs")
+env.Append(LIBS=libs)
diff --git a/platformio/builder/scripts/frameworks/cmsis.py b/platformio/builder/scripts/frameworks/cmsis.py
index 4af5b98a..66bc005c 100644
--- a/platformio/builder/scripts/frameworks/cmsis.py
+++ b/platformio/builder/scripts/frameworks/cmsis.py
@@ -7,7 +7,7 @@
 
 from os.path import join
 
-from SCons.Script import DefaultEnvironment, Return
+from SCons.Script import DefaultEnvironment
 
 env = DefaultEnvironment()
 
@@ -39,4 +39,4 @@ libs.append(envsafe.BuildLibrary(
     join("$PLATFORMFW_DIR", "variants", "${BOARD_OPTIONS['build']['variant']}")
 ))
 
-Return("libs")
+env.Append(LIBS=libs)
diff --git a/platformio/builder/scripts/frameworks/energia.py b/platformio/builder/scripts/frameworks/energia.py
index a4f72a63..8e1e4901 100644
--- a/platformio/builder/scripts/frameworks/energia.py
+++ b/platformio/builder/scripts/frameworks/energia.py
@@ -7,7 +7,7 @@
 
 from os.path import join
 
-from SCons.Script import DefaultEnvironment, Return
+from SCons.Script import DefaultEnvironment
 
 env = DefaultEnvironment()
 
@@ -52,4 +52,4 @@ libs.append(env.BuildLibrary(
     join("$PLATFORMFW_DIR", "cores", "${BOARD_OPTIONS['build']['core']}")
 ))
 
-Return("libs")
+env.Append(LIBS=libs)
diff --git a/platformio/builder/scripts/frameworks/opencm3.py b/platformio/builder/scripts/frameworks/opencm3.py
index 5742429c..330fcd07 100644
--- a/platformio/builder/scripts/frameworks/opencm3.py
+++ b/platformio/builder/scripts/frameworks/opencm3.py
@@ -9,7 +9,7 @@ import re
 from os import listdir, sep, walk
 from os.path import isfile, join, normpath
 
-from SCons.Script import DefaultEnvironment, Return
+from SCons.Script import DefaultEnvironment
 
 from platformio.util import exec_command
 
@@ -168,4 +168,4 @@ libs.append(env.Library(
     get_source_files(root_dir)
 ))
 
-Return("libs")
+env.Append(LIBS=libs)
diff --git a/platformio/builder/scripts/frameworks/spl.py b/platformio/builder/scripts/frameworks/spl.py
index ee19fc0c..b4644678 100644
--- a/platformio/builder/scripts/frameworks/spl.py
+++ b/platformio/builder/scripts/frameworks/spl.py
@@ -7,7 +7,7 @@
 
 from os.path import join
 
-from SCons.Script import DefaultEnvironment, Return
+from SCons.Script import DefaultEnvironment
 
 env = DefaultEnvironment()
 
@@ -61,4 +61,4 @@ libs.append(envsafe.BuildLibrary(
     ignore_files
 ))
 
-Return("libs")
+env.Append(LIBS=libs)
diff --git a/platformio/builder/scripts/teensy.py b/platformio/builder/scripts/teensy.py
index a6f9837c..140fa8fd 100644
--- a/platformio/builder/scripts/teensy.py
+++ b/platformio/builder/scripts/teensy.py
@@ -53,13 +53,11 @@ else:
         UPLOADHEXCMD='"$UPLOADER" $UPLOADERFLAGS'
     )
 
-CORELIBS = env.ProcessGeneral()
-
 #
 # Target: Build executable and linkable firmware
 #
 
-target_elf = env.BuildFirmware(["m"] + CORELIBS)
+target_elf = env.BuildFirmware()
 
 #
 # Target: Build the firmware file
diff --git a/platformio/builder/scripts/timsp430.py b/platformio/builder/scripts/timsp430.py
index 98fbd1d1..90507539 100644
--- a/platformio/builder/scripts/timsp430.py
+++ b/platformio/builder/scripts/timsp430.py
@@ -48,6 +48,8 @@ env.Replace(
         "-Wl,-gc-sections,-u,main"
     ],
 
+    LIBS=["m"],
+
     SIZEPRINTCMD='"$SIZETOOL" -B -d $SOURCES',
 
     UPLOADER=join("$PIOPACKAGES_DIR", "tool-mspdebug", "mspdebug"),
@@ -74,13 +76,11 @@ env.Append(
     )
 )
 
-CORELIBS = env.ProcessGeneral()
-
 #
 # Target: Build executable and linkable firmware
 #
 
-target_elf = env.BuildFirmware(["m"] + CORELIBS)
+target_elf = env.BuildFirmware()
 
 #
 # Target: Build the .hex
diff --git a/platformio/builder/scripts/titiva.py b/platformio/builder/scripts/titiva.py
index b96d4b70..db14ecdf 100644
--- a/platformio/builder/scripts/titiva.py
+++ b/platformio/builder/scripts/titiva.py
@@ -27,13 +27,11 @@ env.Append(
     ]
 )
 
-CORELIBS = env.ProcessGeneral()
-
 #
 # Target: Build executable and linkable firmware
 #
 
-target_elf = env.BuildFirmware(["c", "gcc", "m"] + CORELIBS)
+target_elf = env.BuildFirmware()
 
 #
 # Target: Build the .bin file
diff --git a/platformio/builder/tools/platformio.py b/platformio/builder/tools/platformio.py
index 37931953..e83bf8d4 100644
--- a/platformio/builder/tools/platformio.py
+++ b/platformio/builder/tools/platformio.py
@@ -10,7 +10,7 @@ from SCons.Script import SConscript, SConscriptChdir
 from SCons.Util import case_sensitive_suffixes
 
 
-def ProcessGeneral(env):
+def BuildFirmware(env):
     # fix ASM handling under non-casitive OS
     if not case_sensitive_suffixes('.s', '.S'):
         env.Replace(
@@ -24,20 +24,8 @@ def ProcessGeneral(env):
     if "BUILD_FLAGS" in env:
         env.MergeFlags(env['BUILD_FLAGS'])
 
-    corelibs = []
-    if "FRAMEWORK" in env:
-        if env['FRAMEWORK'] in ("arduino", "energia"):
-            env.ConvertInoToCpp()
-        for f in env['FRAMEWORK'].split(","):
-            SConscriptChdir(0)
-            corelibs += SConscript(
-                env.subst(join("$PIOBUILDER_DIR", "scripts", "frameworks",
-                               "%s.py" % f.strip().lower()))
-            )
-    return corelibs
+    env.BuildFramework()
 
-
-def BuildFirmware(env, corelibs):
     firmenv = env.Clone()
     vdirs = firmenv.VariantDirRecursive(
         join("$BUILD_DIR", "src"), "$PROJECTSRC_DIR")
@@ -59,12 +47,15 @@ def BuildFirmware(env, corelibs):
         _LIBFLAGS=" -Wl,--end-group"
     )
 
-    firmenv.MergeFlags(getenv("PLATFORMIO_SRCBUILD_FLAGS", "$SRCBUILD_FLAGS"))
+    _srcbuild_flags = getenv("PLATFORMIO_SRCBUILD_FLAGS",
+                             env.subst("$SRCBUILD_FLAGS"))
+    if _srcbuild_flags:
+        firmenv.MergeFlags(_srcbuild_flags)
 
     return firmenv.Program(
         join("$BUILD_DIR", "firmware"),
         [firmenv.GlobCXXFiles(vdir) for vdir in vdirs],
-        LIBS=corelibs + deplibs,
+        LIBS=env.get("LIBS") + deplibs,
         LIBPATH="$BUILD_DIR",
         PROGSUFFIX=".elf"
     )
@@ -95,6 +86,21 @@ def VariantDirRecursive(env, variant_dir, src_dir, duplicate=True,
     return variants
 
 
+def BuildFramework(env):
+    if "FRAMEWORK" not in env:
+        return
+
+    if env['FRAMEWORK'].lower() in ("arduino", "energia"):
+        env.ConvertInoToCpp()
+
+    for f in env['FRAMEWORK'].split(","):
+        SConscriptChdir(0)
+        SConscript(
+            env.subst(join("$PIOBUILDER_DIR", "scripts", "frameworks",
+                           "%s.py" % f.strip().lower()))
+        )
+
+
 def BuildLibrary(env, variant_dir, library_dir, ignore_files=None):
     lib = env.Clone()
     vdirs = lib.VariantDirRecursive(
@@ -303,10 +309,10 @@ def exists(_):
 
 
 def generate(env):
-    env.AddMethod(ProcessGeneral)
     env.AddMethod(BuildFirmware)
     env.AddMethod(GlobCXXFiles)
     env.AddMethod(VariantDirRecursive)
+    env.AddMethod(BuildFramework)
     env.AddMethod(BuildLibrary)
     env.AddMethod(BuildDependentLibraries)
     env.AddMethod(ConvertInoToCpp)