include: add SPDX GPL-2.0-only license tags for files without licensing information ...

According to our /README, GPL-2.0-only applies for the whole project
except noted otherwise.

Signed-off-by: Roland Hieber <rohieb@rohieb.name>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add generic uncompress function ...

uncompress() has the same prototype as the various kernel decompress
functions. It automatically detects the compression type and selects
the correct uncompress function.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>