lib/oe/sstatesig: Ensure packagegroups don't continually rebuild

lib/oe/sstatesig: Ensure packagegroups don't continually rebuild

packagegroups are allarch and shouldn't change depending on the target
or machine selected. In general they should have good stable namespaces
for their dependencies. As such we can exclude them from rebuilding when
dependency checksums change.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Saul Wold <sgw@linux.intel.com>