Move SELinux labeling function into selinux-image.bbclass.
Add a test with 'WARNING' output to catch files without SELinux contexts after file labeling.

Currently we have a few files that remain unlabeled that don't break
anything at runtime since they're mount points and the file systems
that get mounted on them are labeled. Still, I've seen some weird
behavior in the past where files were getting labeled wrong and having
this data in my build logs would have been very helpful. Adding it
as a warning may help in debugging if someone stumbles across a related
bug in the future.