arm: Generate proper mmc image

* Stub out makebootable like on u-boot
* Place haiku_loader.efi at EFI/BOOT/BOOTXXX.EFI
* Generate boot.scr for u-boot + EFI
* haiku-mmc.image now attempts to auto-boot in qemu-system-arm
* Same code paths should roughly apply for arm64,riscv,etc

Change-Id: I563ebd77422d9e5dfcfeeedadcbc60471e8a57f4
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2938
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>