sys: net: Initial import of a basic MAC protocol

This is basically an adapter translating `netapi` IPC calls into `netdev` function calls.

Depends on ~~#1448~~ (merged) and ~~#2027~~ (merged)
> And the registrar is the thread the receiving thread is registered to, not the receiving thread. That would be the registrant.

I don't get this. This thread (basic_mac) gets a message of type `NETAPI_CMD_GET` asking for all registered threads (`BASIC_MAC_REGISTRY` == `NETAPI_CONF_REGISTRY'). And now this loop looks for entries in `_basic_mac_registry` that matches `current_pid`. Since this loop is executed from within basic_mac this should be the PID of the basic_mac thread, right?