Package udev config for firmware

@PabloCastellano got the firmware loading working in #143 (closed) but I think that the firmware.sh file and the udev rule can be moved to a seperate package (or postmarketos-base) and modify firmware.sh to also search files in /lib/firmware/postmarketos.

This system should work for drivers following the kernel way of loading firmware and will become more important for mainlined devices.