Add the 'dvd1.iso' target. This mimics the 'release.iso' target,
with the additional step of fetching packages for inclusion on the
dvd image.

The 'pkg-stage' target is used to run 'scripts/pkg-stage.sh' if
the '${TARGET}/pkg-stage.conf' configuration file exists (currently
only amd64 and i386).

Allow dvd1.iso to be skipped if NODVD=1.

MFC after: 3 days
X-MFC-With: r258305, r258307, r258308, r258309
Sponsored by: The FreeBSD Foundation