Fix/sstate handling

Created by: petermarko

Old override syntax is causing huge sstate objects. Fixing the override correctly skips sstate blob creation.

Additionally during review of the sstate handling I found out that setscene function is missing. I'm actually not quite sure why it's needed as my images are always rebuilt, but all image classes using sstate in poky have this. Feel free to move it to other class, I put it just next to the other addtask clauses.

Please cherry-pick this to kirkstone after merging.