Code review comment for ~xnox/ubuntu/+source/u-boot:fixup-fdtdir-properly

The patches all look sane, so +1 from me.

The one thing that worries me is I'm not sure "soc" or "board" are ever *meant* to be blank in the env, but I can't find anything to that effect in the docs/comments. Anyway, they evidently are in this case so the workaround's necessary.