distro_bootcmd: Add support for booting from ubifs

Under the assumptions of having a UBI volume called boot, containing
a ubifs filesystem.

Signed-off-by: Roy Spliet <rspliet@eclipso.eu>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Stephen Warren <swarren@nvidia.com>