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>

.gitmodules

@@ -0,0 +1,5 @@
+# This file maps a submodule path to an url from where the submodule
+# can be obtained. The script "submodules.sh" finds the url in this file
+# when invoked with -i to clone the submodules.
+
+git		git://git.kernel.org/pub/scm/git/git.git