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

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"):