config_distro_bootcmd.h: Prefer booting from bootable paritions

List bootable partitions and only scan those for bootable files, falling
back to partition 1 if there are no bootable partitions

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>