prepare_internal_flashing: add license headers

prepare_internal_flashing.sh

@@ -1,4 +1,6 @@
 #!/bin/bash
+# SPDX-License-Identifier: GPL-3.0+
+# Copyright (C) 2018, Martin Kepplinger <martink@posteo.de>
 RED='\033[0;31m'
 GREEN='\033[0;32m'
 NC='\033[0m'