new: debian docker targets for cross-compiling

new: debian docker targets for cross-compiling

This provides a basic Debian install with access to the emdebian cross
compilers. The debian-armhf-cross and debian-arm64-cross targets build
on the basic Debian image to allow cross compiling to those targets.

A new environment variable (QEMU_CONFIGURE_OPTS) is set as part of the
docker container and passed to the build to specify the
--cross-prefix. The user still calls the build in the usual way, for
example:

  make docker-test-build@debian-arm64-cross \
    TARGET_LIST="aarch64-softmmu,aarch64-linux-user"

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170220105139.21581-3-alex.bennee@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>