release: create pkgbase repo config from Makefile

This will allow the VM image build scripts to use the same repository
config file for installing base system packages at build time.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51482

(cherry picked from commit f4f0308440d2321fa9172dab1029f16a70289e4e)