tools/makemanifest.py: Fix build on Windows by adding .exe to mpy-cross.

When using a manifest on Windows the reference to mpy-cross compiler was
missing the .exe file extension, so add it when appropriate.

Also allow the default path to mpy-cross to be overridden by the (optional)
MICROPY_MPYCROSS environment variable, to allow full flexibility on any OS.

tools/makemanifest.py

@@ -235,6 +235,9 @@ def main():
     # Get paths to tools
     MAKE_FROZEN = VARS["MPY_DIR"] + "/tools/make-frozen.py"
     MPY_CROSS = VARS["MPY_DIR"] + "/mpy-cross/mpy-cross"
+    if sys.platform == "win32":
+        MPY_CROSS += ".exe"
+    MPY_CROSS = os.getenv("MICROPY_MPYCROSS", MPY_CROSS)
     MPY_TOOL = VARS["MPY_DIR"] + "/tools/mpy-tool.py"
 
     # Ensure mpy-cross is built