package/linux-firmware: install missing symlink-target files

AS reported by ovf on IRC, some of the firmwares we install are in fact
symlinks to some other files.

Install those files as well, and document the existing symlinks.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>