roms: add the edk2 project as a git submodule

The roms/edk2 submodule can help with three goals:
- build the OVMF and ArmVirtQemu virtual UEFI firmware platforms (to be
  implemented later),
- build the EfiRom tool on the fly, which is used in roms/Makefile, for
  building the "efirom" target,
- build UEFI test applications (to be run in guests), for qtest support.

Edk2 commit 85588389222a3636baf0f9ed8227f2434af4c3f9 stands for the latest
"stable tag", namely "edk2-stable201811".

The edk2 repository tracks some binary files that should not be removed by
QEMU's top-level "make clean"; exempt the full pathnames from the "find"
command.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Shannon Zhao <shannon.zhaosl@gmail.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20190204160325.4914-2-lersek@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>