Comments: FreeBSD.conf should not be overwritten

The files /usr/sbin/pkg/FreeBSD.conf.* currently set base repo to {enable: no}.
We don't want to overwrite /local/etc/pkg/repos/FreeBSD.conf that already exists
when users choose tech preview in the installation.

Signed-off-by: Guilherme Augusto de Souza Candinho <cndghm@gmail.com>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1925