safcm: group_order: higher priority for listed groups over remaining groups

The priority for all groups listed in `group_order` was properly
respected: Early entries had the highest priority. However, groups which
were not listed in `group_order` had a higher priority and overwrote
files from all groups configured in `group_order`.

The priority is now as expected (from high to low): host itself, groups
in group_order (from high to low), remaining groups.