armv8: fix boot into qemu with make target

Signed-off-by: Lukas Humbel <lukas.humbel@inf.ethz.ch>

harness: Add EFI image tool to create EFI disk images

This can be used to create a bootable EFI medium.

Also adds a simple command-line tool to create an EFI disk.

Signed-off-by: Moritz Hoffmann <moritz.hoffmann@inf.ethz.ch>