Use the new debian-s390x-cross.docker target to cross compile for
s390.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20170227143028.16428-3-alex.bennee@linaro.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

Ostensibly Shippable offers a similar set of services as Travis.
However they are focused on Docker container based work-flows so we
can use our existing containers to run a few extra builds - in this
case a bunch of cross-compiled targets on a Debian multiarch system.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170220105139.21581-4-alex.bennee@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>