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>