externalsrc: Detect code changes in submodules

Further to 50ff9afb, only detect code changes in submodules that are
subdirectories of the EXTERNALSRC directory.

git submodule status returns a path relative to the cwd for each submodule.
We don't add submodules that are not within our source subtree.

Signed-off-by: Douglas Royds <douglas.royds@taitradio.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 4525310d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>