Add grub EFI support

Add grub EFI support

Note that we only generate the necessary menu entries for the grub
configuration to chainload into our generated UKIs, we do not yet
install grub for EFI ourselves as this is a distribution specific
mess that we still need to figure out. On Fedora, because the
shim and grub2-efi packages install directly to /boot which we
redirect to /efi, this is sufficient to boot with grub on EFI by
simply installing the shim and grub2-efi packages.

For other distributions, a post install or finalize script will be
necessary that installs grub (and optionally shim) to the correct
locations in the ESP.