Improve debugging in debug_load_mode = modified and fix an issue with useless project rebuilding

HISTORY.rst

@@ -7,6 +7,7 @@ PlatformIO 3.0
 3.6.7 (2019-??-??)
 ~~~~~~~~~~~~~~~~~~
 
+* `PIO Unified Debugger <https://docs.platformio.org/en/page/plus/debugging.html>`__: improved debugging in ``debug_load_mode = modified`` and fixed an issue with useless project rebuilding
 * Project Generator: fixed a VSCode C/C++'s "Cannot find" warning when CPPPATH folder does not exist
 * Fixed an "IndexError: list index out of range" for Arduino sketch preprocessor
   (`issue #2268 <https://github.com/platformio/platformio-core/issues/2268>`_)

platformio/builder/tools/piomisc.py

@@ -296,8 +296,7 @@ def ProcessDebug(env):
     if not env.subst("$PIODEBUGFLAGS"):
         env.Replace(PIODEBUGFLAGS=["-Og", "-g3", "-ggdb3"])
     env.Append(
-        PIODEBUGFLAGS=["-D__PLATFORMIO_DEBUG__"],
-        BUILD_FLAGS=env.get("PIODEBUGFLAGS", []))
+        BUILD_FLAGS=list(env['PIODEBUGFLAGS']) + ["-D__PLATFORMIO_DEBUG__"])
     unflags = ["-Os"]
     for level in [0, 1, 2]:
         for flag in ("O", "g", "ggdb"):