oelint.vars.bbclassextend: don't warn on image or packagegroup

[amuetzel]

Another tiny fix/enhancement to reduce clutter in the output:

Usually, I think one would probably not try to build a -native or -nativesdk variant of an image or a packagegroup recipe. So skip this warning in these cases.

This is the same logic as in rule_file_underscores.py, which also skips these two kinds of recipes.

Pull request checklist

Bugfix

• [x] A testcase was added to test the behavior

[amuetzel]

Sorry for bothering you with such tiny fixes ;-) If I find more similar situations, there might be more PRs coming up. So from my side, there's no need to rush a new release.