firmware: Allow binary files to be loaded by /boot/loader

Files loaded "-t firmware" (or module_type="firmware"). They are
registered with the firmware system using the full path to the file.
There's only one firmware per file, and it is the entire file. We do an
extra firmware_get() on any firmware we find here to prevent them from
ever being unloaded (we can't handle that case sanely).

Sponsored by: Netflix
Reviewed by: tsoome, jhb
Differential Revision: https://reviews.freebsd.org/D43522