release: Add -DPKGBASE option to include pkgbase packages

If this option is set, an offline repo of pkgbase packages corresponding
to base.txz and kernel.txz will be included in the disc1 release media
rather than the base.txz and kernel.txz tarballs.

Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50346