Add COPYING/License file to omx-il

As I read it from the SPDX identifiers in the soft_3rdpart/omx-il code folder, all of the code there is GPL-2.0. It would be great if you could also add a COPYING file with this information into this folder to make it more visible, or even better, put a REUSE.software compatible ./LICENSES/GPL-2.0-only.txt file there.