tests/docker: update and flatten debian-all-test-cross

tests/docker: update and flatten debian-all-test-cross

Update to the latest stable Debian. While we are at it flatten into a
single dockerfile. We also need to ensure we install clang as it is
used for those builds as well.

It would be nice to port this to lcitool but for now this will do.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220914155950.804707-24-alex.bennee@linaro.org>