release: create pkgbase VM and cloud images

VM and cloud images are now built as packaged base systems by default,
matching the default for installation media.

Setting -DNOPKGBASE allows building as non-pkgbase systems.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51483

(cherry picked from commit 4227d51b6e09ceaaae0f5832c85ec700d80ef18f)