cpu/stm32/gen_kconfig: move script license to SPDX format

Author: krzysztof-cabaj

cpu/stm32/dist/kconfig/gen_kconfig.py

@@ -1,10 +1,7 @@
 #!/usr/bin/env python3
 
-# Copyright (C) 2020 Inria
-#
-# This file is subject to the terms and conditions of the GNU Lesser
-# General Public License v2.1. See the file LICENSE in the top level
-# directory for more details.
+# SPDX-FileCopyrightText: 2020 Inria
+# SPDX-License-Identifier: LGPL-2.1-only
 
 import os
 import argparse