Adds hardware.raspberry-pi.firmware to stage the Pi's pre-Linux boot files (GPU boot code, DTBs, overlays, config.txt, optionally U-Boot) onto the firmware partition. Boards still boot via generic-extlinux-compatible; this only supplies the files it needs, through populateCmd for image builders or an opt-in activation script.
DTB/overlay copy adapted from nvmd/nixos-raspberrypi.
