mynewt: allow newt to bypass submodule cloning

A recently added `newt` feature allows it to only clone selected git
submodules: https://github.com/apache/mynewt-newt/pull/377

. This changes
the MCUBoot repository to remove submodules from the cloning process,
because they are not used by Mynewt.

Signed-off-by: Fabio Utzig <utzig@apache.org>