dist: add Dockerfile for docker builder container
Merge pull request #1637 from kaspar030/docker

dist: add Dockerfile for docker builder container
sys: lib: introduce ringbuffer_remove()

This commit adds a function that deletes n elements from the ringbuffer
without actually writing them somewhere.
core: ipc: introduce new messaging API
sys: add msg_queue module
adapt to new msg_queue API
tests: sizeof_tcb: adapt to msgless tcb