linux-firmware: add LICENSE_FILES and LICENSE file handling

linux-firmware: add LICENSE_FILES and LICENSE file handling

The firmware blobs have all different licenses from the different
manufacturers of the binary blobs. This information is contained in the
upstream 'linux-firmware' repositroy.

This commit extends the package handling so that this information can be
added as an additional argument during packages generation.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
(cherry picked from commit 5c14de1d7e715ef882f34925ee6702b847efcbbd)
Link: https://github.com/openwrt/openwrt/pull/15918
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>