reuse existing git clone

This is almost identical to the ABS suggested way of dealing with git, but
keeping the option "--depth 1" to speed up cloning.

Without this change, a subsequent run will fail on git clone because the target
directory already exists.
add missing USB_NET modules

QMI_WWAN has been added as a module to the default raspi config in commit:

b7c5c10f93a4f48609c3e8decaa37ab5aa04d830
on
git://github.com/raspberrypi/linux.git