post-install packaging cloud provider images.
Add a 'gce-package.sh' script to generate the final output
image ready for upload to the GCE platform. Right now, this
is the only image that has a specific output format (GNU-tar),
and this implementation is expected to be temporary.
This is not directly connected to the other release targets.
MFC after: 1 week
X-MFC-with: r279249, r279250
Sponsored by: The FreeBSD Foundation
dc1c45001a