Rename DOCKER_INTERMEDIATE_IMAGES to DOCKER_PARTIAL_IMAGES and add the
incomplete cross compiler images that can build tests but can't build
QEMU itself. We also add debian, debian-bootstrap and the tricode
images to the list.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>