docker/dockerfiles/debian-s390-cross: include clang

It's a silly little limitation on Shippable that is looks for clang
in the container even though we won't use it. The arm/aarch64 cross
builds inherit this from debian.docker but as we needed to use
debian-testing for this we add it here. We also collapse the update
step into one RUN line to remove and intermediate layer of the docker
build.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <20170306112848.659-1-alex.bennee@linaro.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/docker/dockerfiles/debian-s390x-cross.docker

@@ -13,8 +13,8 @@ RUN cat /etc/apt/sources.list | sed "s/deb/deb-src/" >> /etc/apt/sources.list
 RUN dpkg --add-architecture s390x
 
 # Grab the updated list of packages
-RUN apt update
-RUN apt dist-upgrade -yy
+RUN apt update && apt dist-upgrade -yy
+RUN apt install -yy build-essential clang
 RUN apt-get build-dep -yy -a s390x qemu || apt-get -f install
 RUN apt install -yy gcc-multilib-s390x-linux-gnu binutils-multiarch