Create rpi- prefixed symlinks in /usr/local/bin

(eg. rpi-gcc, rpi-ld)
Both user & root paths use same su command

Partially to remove duplicated code paths, but mainly to make sure that
there are no subtle differences hiding away.
Use sudo instead of su in entrypoint.sh
Environment vars get passed through to sudo cmd
Split apt-get list

As per:

  https://docs.docker.com/articles/dockerfile_best-practices/#sort-multi-line-arguments
Split docker.sh into rpi-xc and build.sh
Update README.md