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)
I think this poses a problem. From what I understood, if I want to register an upper layer for this nomac layer I have to send a `NETAPI_CMD_REG` msg to nomac's running thread. The command has to contain the `kernel_pid_t` of my upper layer thread in the `reg_pid` field. Nomac's thread then fills a `_nomac_registry entry` with:
`_nomac_registry[i].registrar_pid = thread_getpid();`
`_nomac_registry[i].recipient_pid = reg->reg_pid;`
At this point `thread_getpid()` is the pid of nomac's running thread.

Now let's assume we receive a packet on the device that was registered for the nomac thread. `_nomac_recv_cb` is called and we use `kernel_pid_t current_pid = thread_getpid();` to determine the pid and look for entries in the registry. But this shouldn't work because at this point `thread_getpid()` does not return the pid of nomac's thread! `_nomac_recv_cb` is processed in a different process (the one where the receive of the device is triggered) and the equality check `_nomac_registry[i].registrar_pid == current_pid` fails.