firmware: load binary firmware files

When we can't find a .ko module to satisfy the firmware request, try
harder by looking for a file to read in directly. We compose this file's
name by appending the imagename parameter to the firmware path
(currently hard-wired to be /boot/firmware, future plans are for a
path). Allow this file to be unloaded when firmware_put() releases the
last reference, but we don't need to do the indirection and dance we
need to do when unloading the .ko that will unregister the firmware.

Sponsored by: Netflix
Reviewed by: manu, jhb
Differential Revision: https://reviews.freebsd.org/D43555