externalsrc: Detect code changes in submodules

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

The (undocumented) git submodule--helper returns a path
for each submodule relative to the top of the repo.
Don't add submodules that are not within our source subtree.

[YOCTO #14333]

Signed-off-by: Douglas Royds <douglas.royds@taitradio.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>