Create images that use the Raspberry Pi kernel + bootloader (instead of generic linux-aarch64 + U-Boot)

[fwcd]

There still seem to be some issues with the mainline kernel + U-Boot, therefore it would be nice to have ready-to-use images that use the Pi Foundation's kernel and bootloader.

See https://forum.endeavouros.com/t/custom-kernels-on-raspi-4/39059/3, we'd need to do something along these lines in a script:

pacman -R --noconfirm linux-aarch64 uboot-raspberrypi
pacman -Syu --noconfirm --needed linux-rpi raspberrypi-bootloader raspberrypi-firmware
cp /boot/config.txt /boot/config.txt.orig
cp /home/alarm/configs/rpi4-config.txt /boot/config.txt