diff --git a/platformio/__init__.py b/platformio/__init__.py
index 7b30378b..4a829ad3 100644
--- a/platformio/__init__.py
+++ b/platformio/__init__.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-VERSION = (2, 3, "6.dev2")
+VERSION = (2, 3, "6.dev3")
 __version__ = ".".join([str(s) for s in VERSION])
 
 __title__ = "platformio"
diff --git a/platformio/builder/scripts/frameworks/mbed.py b/platformio/builder/scripts/frameworks/mbed.py
index a5f9b76e..a0454b07 100644
--- a/platformio/builder/scripts/frameworks/mbed.py
+++ b/platformio/builder/scripts/frameworks/mbed.py
@@ -29,7 +29,7 @@ http://mbed.org/
 import re
 import xml.etree.ElementTree as ElementTree
 from binascii import crc32
-from os import walk
+from os import getenv, walk
 from os.path import basename, isfile, join, normpath
 
 from SCons.Script import DefaultEnvironment, Exit
@@ -226,7 +226,11 @@ env.Replace(
 )
 
 # restore external build flags
-env.ProcessFlags()
+env.ProcessFlags([
+    env.get("BOARD_OPTIONS", {}).get("build", {}).get("extra_flags", None),
+    env.get("BUILD_FLAGS"),
+    getenv("PLATFORMIO_BUILD_FLAGS", None),
+])
 
 # Hook for K64F and K22F
 if board_type in ("frdm_k22f", "frdm_k64f"):