FiXos kernel have no explicit license!

Yes, this is a shame! I never really added a license on FiXos' source code, because of some hesitations between permissive or copylefted one, at the time. But for this case, no choice is maybe worst than 'bad' choice, so we need to decide in the next weeks.

I hesitated between GPLv3, modified BSD one, and Creative Common CC0, mainly. Currently, my preference is for GPLv3, but I would like to see if anybody is interested to discuss this point. Please feel free to give your opinion!

(I'll try to add some pros/cons in the case of this project for each one quickly!)