roms: build the EfiRom utility from the roms/edk2 submodule

roms: build the EfiRom utility from the roms/edk2 submodule

Building the EfiRom utility from "roms/edk2/BaseTools" should make
"roms/Makefile" more self-contained. Otherwise, we'd call the system-wide
EfiRom for building the combined iPXE option ROMs, but call the sibling
utilities from "roms/edk2/BaseTools" for building "roms/edk2" content.

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>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190204160325.4914-3-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>