Resolve SSL verification issue at shippable container's git_sync stage:

shippable logs:
--------------
git_sync
- ssh-agent bash -c 'ssh-add /tmp/ssh/01_deploy; git clone https://github.com/philmd/qemu.git /root/src/github.com/philmd/qemu'
Identity added: /tmp/ssh/01_deploy (rsa w/o comment)
Cloning into '/root/src/github.com/philmd/qemu'...
fatal: unable to access 'https://github.com/philmd/qemu.git/': Problem with the SSL CA cert (path? access rights?)
retrying 1 of 3 times...
Suggested-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
[AJB: fixed re-base conflict following stretch updates]
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

Debian has now released Stretch as its new stable. As we track
debian:stable-slim this has a few consequences. For one thing we can
now drop the emdebian hacks as cross compilers are part of the
official repositories now. However we do loose the ability to build
against powerpc (not ppc64) since that is no longer a release
architecture.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

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