SPDX moved from GPL-2.0 to GPL-2.0-only and from GPL-2.0+ to
GPL-2.0-or-later. Reflect that in the SPDX license headers.
Signed-off-by: Paul Spooren <mail@aparcar.org>
[adjust commit title, update remaining files]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>