device/testing/firmware-*: move firmware to /lib/firmware/postmarketos
We keep device-specific firmware in this directory and not in /lib/firmware.
Tested on lg-hammerhead.
There are still a couple of files left in /lib/firmware then:
- linux-t2m-flame: http://pkgs.postmarketos.org/contents?file=&path=%2Flib%2Ffirmware&name=%09linux-t2m-flame&branch=master
- firmware-analogix: http://pkgs.postmarketos.org/contents?file=&path=%2Flib%2Ffirmware&name=firmware-analogix&branch=master - can remain that way imo