not enabled yet, but uncommenting the grub_mod should enable
building of isolinux+grubefi hybrid images. once uefi boot works
and is verified on aarch64, we can enable the vanilla iso image
(grsec does not support aarch64).
the boot repositry needs to be signed with a key. We explicitly copy
this to initramfs so users don't need use --hostkeys which requires
access to /etc/apk/keys/
without the key in intramfs the boot repository will be useless
