kbuild: Fixup deb-pkg target to generate separate firmware deb

The below is a simplistic fix for "make deb-pkg"; it splits the
firmware out to a linux-firmware-image package and adds an
(unversioned) Suggests to the linux package for this firmware.

Signed-Off-By: Jonathan McDowell <noodles@earth.li>
Acked-by: Frans Pop <elendil@planet.nl>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>