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)
Otherwise, if I understood @haukepetersen correctly, the thread "where the receive of the device is triggered" would always be the ISR, which - considering that the ISR should be running only for a few instructions - would be bad.