core: sys: move ringbuffer to core
Merge pull request #2017 from kaspar030/move_ringbuffer_to_core

Move ringbuffer to core
core: ipc: introduce new messaging API
adapt to new msg_queue API
tests: sizeof_tcb: adapt to new IPC
tests: add msg_node example/test
core: remove cib

The "circular integer buffer" was only used by the old message queues.
So remove it for now.