release: Make PKGBASE the default

Replace the PKGBASE option by a NOPKGBASE option.

Check for both whether the option is defined and whether it is empty,
since when running from release.sh we always define NOPKGBASE.

MFC after: 12 hours
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D52536

(cherry picked from commit 35b81af1eee699509bae2bbe12fdc770d9cee40a)