gitlab-ci.yml: Add jobs to build EDK2 firmware binaries

Add two GitLab job to build the EDK2 firmware binaries.

The first job build a Docker image with the packages requisite
to build EDK2, and store this image in the GitLab registry.
The second job pull the image from the registry and build the
EDK2 firmware binaries.

The docker image is only rebuilt if the GitLab YAML or the
Dockerfile is updated.
The second job is only built when the roms/edk2/ submodule is
updated, when a git-ref starts with 'edk2' or when the last
commit contains 'EDK2'. The files generated are archived in
the artifacts.zip file.

With edk2-stable201905, it took 2 minutes 52 seconds to build
the docker image, and 36 minutes 28 seconds to generate the
artifacts.zip with the firmware binaries (filesize: 10MiB).

See: https://gitlab.com/philmd/qemu/pipelines/107553178



Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>