OpenWrt ships code not only licensed via GPL-2.0-{only,or-later}.
Reflect that by adding the missing licenses, just like the kernel
does.

This patch takes the license text as it is found in the Linux
kernel, which might probably be the best common denominator here.

In contrast to the kernel versions, only some changes to the
valid and recommended identifiers were made (GPL-x.x+ vs.
GPL-x.x-or-later and similar).

Suggested-by: Paul Spooren <mail@aparcar.org>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>