netdev: Provide test network device with netdev interface

The reasoning behind this PR is twofold:

1. test the "lower" part (the device part) of netdev
2. provide a well defined testing infrastructure for modules that utilises netdev

To test this, run

```
make -C tests/unittests tests-netdev_dummy flash term
```

~~Depends on #1492:~~ (it's merged)

```
* #1680
|\
… …
| |
| * #1448
| |\
* | | #1733
| * | #1732
| | * #1731
| |/
|/
* (master)
```


`netdev_dummy` is a module that is only really useful in the context of unittests. So why not define it's dependencies in the Makefile of unittests?