Conwayste Cross-Compiling Docker files

(making custom Docker images for cross, used by our CI)[https://github.com/rust-embedded/cross#custom-docker-images]

$ TAG=aaronm04/cross:x86_64-unknown-linux-gnu    # change the part after the colon according to .travis.yml matrix
$ cd linux
$ docker build -t $TAG .
$ docker push $TAG