Draft: feat: Add aarch64 efi firmware target

Adds a genrule to generate aarch64 EFI firmware, which is required to boot qemu-system-aarch64.

This generates the files flash0.img and flash1.img, which can be used to boot an aarch64 instance as follows:

qemu-system-aarch64 -nographic -machine virt,gic-version=max -m 512M -cpu max -smp 4 \
-netdev user,id=vnet,hostfwd=:127.0.0.1:0-:22 -device virtio-net-pci,netdev=vnet \
-drive file=<image>,if=none,id=drive0,cache=writeback -device virtio-blk,drive=drive0,bootindex=0 \
-drive file=flash0.img,format=raw,if=pflash -drive file=flash1.img,format=raw,if=pflash 

Reference guide