driver: netdev: cc2420: Move radio_driver to netdev_802154 & Port cc2420 accordingly

To test this, run

```
export BOARD=telosb # or z1, or wsn430-v1_4
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)
```
How did that happen? oO