west: sign.py: add "REM" support to pass comments through cpp

Generated outputs can be difficult to read, preserving comments helps a
lot and they often provide good `git grep` search keywords.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

scripts/west_commands/sign.py

@@ -444,6 +444,12 @@ def preprocess_toml(self, config_dir, toml_basename, subdir):
         # unfortunately don't seem to make any difference here and they're
         # gcc-specific)
         preproc_cmd += ['-P']
+        # REM allows standalone comments to be preserved in generated/*.toml.
+        # A "double" `REM # my comment...` is not necessary but recommended for
+        # syntax highlighting.
+        # Note trailing comments are ignored by cpp and don't need any REM
+        # trick, a trailing `#` is enough.
+        preproc_cmd += ['-DREM=#']
         preproc_cmd += ['-I', str(self.sof_src_dir / 'src')]
         preproc_cmd += ['-imacros',
                         str(pathlib.Path('zephyr') / 'include' / 'generated' / 'autoconf.h')]