freebsd-update: Use BASEDIR when checking for src componentfreebsd-update: Use BMFC r343152: freebsd-update: Use BASEDIR when checking for src component

src could potentially be installed under the based dir
and not under the root or vice versa.

PR: 224048
Submitted by: Gerald Aryeetey <aryeeteygerald_rogers.com>
Reviewed by: delphij
Sponsored by: The FreeBSD Foundation