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)
```
The change in https://github.com/RIOT-OS/RIOT/pull/1732/files#diff-2b35e3dab26520778e1991f7041d00a5R3 seems to imply it is still possible to use nativenet without netdev_base..?