Add support for UEFI executable signing
Created by: gekmihesg
Allows setting ALL_efi_cert
/<preset>_efi_cert
and ALL_efi_key
/<preset>_efi_key
in presets to automatically sign generated EFI executables.
At the moment there doesn't seem to be a clean way to do this otherwise. Using a pacman hook only partially works and would probably leave the user with a unbootable system when running mkinitcpio
manually.