Fill the gap between MirageOS and Gilbraltar

[dinosaure]

a mirage-gilbraltar, as a mirage-solo5 should exists and we should extend targets inside the mirage/mirage project. However, I think we must implement yield before to be able to launch lwt.

[dinosaure]

Currently, we have a mirage-gilbraltar and a PR is already done on the mirage side: mirage/mirage#1253. However, I will consider that everything will be done if we can bring our mirage-tcpip stack into gilbraltar via mirage configure -t rpi4. In that case, the gap will be filled!