nativenet: Port nativenet for netdev

To test this, run

```
export BOARD=native
SENDER=0 make -C tests/netdev/ all flash && SENDER=1 make -C tests/netdev/ all flash && make -C tests/netdev/ test
```

~~Depends on #1492:~~ (it's merged)

```
* #1680
|\
… …
| |
| * #1448
| |\
* | | #1733
| * | #1732
| | * #1731
| |/
|/
* (master)
```
I'm wondering about two things here:
- why not use it in `tests/netdev/main.c`?
- wouldn't it make more sense to set this from the device?