License headers

This is a derivative work of OpenWrt, all the work should be released under the same license: GPLv2 only.

Code files should contain an header with valid SPDX ID and a copyright statement.

Example for sh, bash, python and Makefile files:

#
# Copyright (C) 2024 Nethesis S.r.l.
# SPDX-License-Identifier: GPL-2.0-only
#

Extra packages could be released under a different license.