ltsi/scripts: Add usage information to generate_git

The README states there is a --help argument to the script but non
existed, so here it is. The START environment variable isn't working
properly (or I don't know how to properly use it) so it has not been
included. Autoresume is probably what you want anyways.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>