Add submodules.sh and use it during builds

This adds a shell script which can be be used to initialize, list and
update submodules in a git repository. It reads the file .gitmodules
to find a mapping between submodule path and repository url for the
initial clone of all submodules.

The script is used during cgit builds to enable automatic download and
checkout of the git git repository.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>