.travis.yml: pre-seed sub-modules for speed

.travis.yml: pre-seed sub-modules for speed

A significant portion of the build time is spent initialising all the
sub-modules we use in the source tree. Often this is almost as long as
the build itself. By pre-seeding the .git/modules tree this will
hopefully improve things.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Brian Jackson <iggy@theiggy.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>