docker: update qemu:debian base following stretch release

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: Alex Bennée <alex.bennee@linaro.org>
