selftest: add a test for gpl3-free images

selftest: add a test for gpl3-free images

Existing tests check that adding a specific gpl3 package (bash)
to core-image-minimal results in expected behaviour.

These tests check the ability to build two common images
without gpl3 components in them:

1. core-image-minimal needs no further tweaks and works
out of the box.

2. core-image-full-cmdline requires dropping the GNU packages
that it pulls in; for good measure this tweaked image is
verified with runtime tests.

These two tests allow dropping meta-gplv2 from being tested
on the autobuilder, however there should be a community consensus
first.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>