Add a special GCE_LICENSE variable to Makefile.gce, which when set,
will include license metadata in the resultant GCE image.

GCE_LICENSE is unset by default, as it primarily pertains to images
produced by the FreeBSD Project, but for downstream FreeBSD consumers,
it can be set in the make(1) environment in the format of:

--licenses="projects/PROJECT_ID/global/licenses/LICENSE_NAME"

The "license" is not a license, per se, but required metadata that
is required by the GCE marketplace. For the FreeBSD Project, the
license name is simply 'freebsd', with the description of 'FreeBSD'.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation