tools/manifestfile.py: Add --unix-ffi option.

Follow up to 35dd959133, allows explicitly
adding the unix-ffi library path from the command line.

This option is needed when building unix-ffi manifests in micropython-lib
CI.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

tools/manifestfile.py

@@ -603,6 +603,9 @@ def main():
         default=os.path.join(os.path.dirname(__file__), "../lib/micropython-lib"),
         help="path to micropython-lib repo",
     )
+    cmd_parser.add_argument(
+        "--unix-ffi", action="store_true", help="prepend unix-ffi to the library path"
+    )
     cmd_parser.add_argument("--port", default=None, help="path to port dir")
     cmd_parser.add_argument("--board", default=None, help="path to board dir")
     cmd_parser.add_argument(
@@ -632,6 +635,8 @@ def main():
         exit(1)
 
     m = ManifestFile(mode, path_vars)
+    if args.unix_ffi:
+        m.add_library("unix-ffi", os.path.join("$(MPY_LIB_DIR)", "unix-ffi"), prepend=True)
     for manifest_file in args.files:
         try:
             m.execute(manifest_file)