Support builds for esp8266

Damien would like to test esp8266 on octoprobe. So I will have a quick look if I can create a container. I try to create a container based on https://github.com/micropython/micropython/tree/master/ports/esp8266#readme which refers to this container: https://github.com/larsks/docker-image-esp-open-sdk. If there is another reason why this build is not supported by mpbuild - please let me know.