sstatesig.py: Move package exclusion list to the layer config
its desireable for other layers to be able to append to the list of packages
with 'safe ABI's which are excluded from the sstate signatures.
I can't emphasise enough how careful you need to be with this list, anything
excluded here needs to be things which don't change interface and are consistent
between different machines.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
