vmimage.subr: Enable FreeBSD-base repo if pkgbase

When creating a VM image using pkgbase, create a configuration file in
/usr/local/etc/pkg/repos/FreeBSD.conf which enables the FreeBSD-base
repository. (This repository is defined in /etc/pkg/FreeBSD.conf as
being disabled by default.)

Approved by: re (cperciva)
Reported by: Mark Millard
MFC after: immediately (needed for 15.0-RC1)

(cherry picked from commit e0c41af9256b5c5a6c97c85d468ff734e29f1bd5)
(cherry picked from commit aade340587336218b32285d50b0e86c463f273e1)