idf.py: Make the lambda function Python 2 & 3 compatible

tools/idf.py

@@ -357,7 +357,7 @@ def print_closing_message(args):
         else:  # flashing the whole project
             cmd = " ".join(flasher_args["write_flash_args"]) + " "
             flash_items = sorted(((o,f) for (o,f) in flasher_args["flash_files"].items() if len(o) > 0),
-                                 key = lambda (o,_): int(o, 0))
+                                 key = lambda x: int(x[0], 0))
             for o,f in flash_items:
                 cmd += o + " " + flasher_path(f) + " "