We need a better solution for OVMF URLs

[HadrienG2]

Currently, the ovmf.sh CI script contains a URL which must be updated frequently. There has to be a better way.

[GabrielMajeri]

Yeah, I also don't like using kraxel's bandwidth for our CI.

I've created a new empty orphan branch, with a separate history from the code, to store the OVMF firmware using Git Large File Storage.

Travis should support Git LFS, and I think it's enough to do something like: checkout the ovmf branch, copy the files, then checkout the original branch.