Create and use uefi-x64.grub.* boot modes
After !216 is merged, we'll have 3 different boot loaders in the ISO:
- syslinux for BIOS,
- systemd-boot for x64 UEFI,
- GRUB for IA32 UEFI (#107).
Since using GRUB allows to solve #63, it looks like replacing systemd-boot with GRUB is currently the only viable option.
