sys: net: Initial import of a simple data-forwarding MAC protocol (nomac)

This is basically an adapter translating `netapi` IPC calls into `netdev` function calls.

Depends on ~~#1448~~ (merged) and ~~#2027~~ (merged)
No. `netdev` is not designed to have the device running in a thread. All direct interactions through `netdev` are via function call (and thus it runs in the same thread as the MAC protocol which manages the device). There are however corner cases (as the above) where the ISR (or in case nativenet a UNIX signal handling thread) react to events (as for example an RX event). To get the follow-up actions to this event (e.g. copying data from device internal buffer to `pktbuf`, demultiplexing packet types, calling receive callback etc.) out of the context of the ISR and into the MAC thread the above sequence is necessary.