UEFI images currently fail on `bootctl install`:

  bootctl --path=/boot/efi install | Failed to get machine id: No medium found
  Action `Install UEFI bootloader` failed at stage Run, error: exit status 1

This is due to bootctl using the machine-id to set the default entry in
/boot/loader/loader.conf and the kernel postinst script using the same value to
create the matching /boot/loader/entries entry.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>