The license folder is a core part of OpenWrt and all GPL-2.0 licensed.
Use SPDX license tags to allow machines to check licenses.

Signed-off-by: Paul Spooren <mail@aparcar.org>
[rebase, keep some Copyright lines, sharpen commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>