rpm-pkg: install firmware files in kernel relative directory

Prior to this patch, firmware files were being installed
in /lib/firmware with a potential conflict with already
installed kernels.

firmware files are now installed in /lib/firmware/<release>
and packaged with the rest of the modules.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>