core_generate_update_payload: initial commit

Takes a filesystem image and generates a update.gz that is signed by N 
provided private keys and verified against the N public keys. This is so we
can do keyrotations.
This pile of goo can be simplified since we don't need to run this on a bare coreos install, just in the SDK or in the update tools zip file. core_roller_upload would be a better template to start from.