externalsrc: git submodule--helper list unsupported

externalsrc: git submodule--helper list unsupported

Git has removed support for "git submodule--helper list".
https://github.com/git/git/commit/31955475d1c283120d5d84247eb3fd55d9f5fdd9

This change provides an alternate method for gathering the submodules
information.

Tested:
Build recipes with and without submodules

Signed-off-by: Carson Labrado <clabrado@google.com>
Signed-off-by: John Edward Broadbent <jebr@google.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6d9364e5f3535954f65cbbc694ee7933ac1d664f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>