tools/makemanifest.py: Expand the path only for known directories.

That imposes the least side effects and does not add a naming
convention. If a manifest.py script want to expand a path, it can do
so using e.g. os.path.abspath().

Signed-off-by: robert-hh <robert@hammelrath.com>

tools/makemanifest.py

@@ -143,7 +143,7 @@ def main():
     # Extract variables for substitution.
     for var in args.var:
         name, value = var.split("=", 1)
-        if os.path.exists(value):
+        if name in ("MPY_DIR", "MPY_LIB_DIR", "BOARD_DIR", "PORT_DIR") and os.path.exists(value):
             value = os.path.abspath(value)
         VARS[name] = value