devtool/friends: Use LAYERSERIES_CORENAMES when generating LAYERSERIES_COMPAT entries

It seems some layers want to subvert the intent of LAYERSERIES_COMPAT
so bitbake is going to have to become stricter about the values there.
To work with this, use LAYERSERIES_CORENAMES to generate the entries in
LAYERSERIES_COMPAT instead of the current magic LAYERSERIES_COMPAT_core
value which may not continue to work.

The downside to this is when migating between releases, people would
need to update devtool workspace layer.conf files. I guess you could
argue this is a feature!

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>