pkg(7): support configuration overwrite like pkg(8)

with pkg(8) it is possible to overwrite a configuration like adding
FreeBSD {
enabled: false
}

in /usr/local/etc/pkg/repo/overwrite.conf which allows to change any
value which can have been reviously configured in anything in
/etc/pkg/*.conf

now the bootstrap supports the same

MFC After: 3 weeks

(cherry picked from commit 5c341fe5123d4aa6961066542de63dd4431d004d)