multiboot2: Change ARMv8 loading to multiboot2.

This might still require some rework as there are slight
dependencies on EFI. It would be better to use the multiboot2
mmap or something custom instead of the raw mmap from EFI.

Signed-off-by: Moritz Hoffmann <moritz@dhcp-16-111-6-143.labs.hpecorp.net>