fix(coreos-base/update_engine): Install update signing key via ebuild.

This removes the need to perform this step during build_image.
feat(coreos-base/coreos-au-key): Install signing key in SDK

This just means tools looking for the test key don't need to point at
the long and screwy path into the coreos-overlay repo.
bump(coreos-base/cros-devutils): Sign updates generated by devserver
Merge pull request #570 from marineam/key

Install update keys via ebuild