Add initial support for automatically uploading GCE virtual

Add initial support for automatically uploading GCE virtual
machine images to the Google Compute Engine platform.

By default, gcutil/gsutil requires an Oauth2 login generated
from a URL that must be opened in a browser, a verification
code copied back to the terminal from which it was invoked,
etc., etc., making it near impossible for automation.

I've hacked together an evil solution to work around this,
so unless GCE_LOGIN_SKIP is set to a non-empty value, this
Makefile will not do anything useful.

As a result of this commit, remove the gce-package.sh script
that was never, nor will ever be, used.

MFC after:	3 days
X-MFC-Note:	(hopefully)
Sponsored by:	The FreeBSD Foundation