drivers: Provide basic interface for all network devices
drivers: Make general IEEE 802.15.4 driver part of net_dev
nativenet: Apply coding conventions
nativenet: Supply net_dev_t support for nativenet
net_dev: Provide some default values dependent on module selection
unittests: add test network device

The thinking behind this commit is twofold:
1. test the "lower" part (the device part) of net_dev
2. provide a well defined testing infrastructure for modules that utilise
   net_dev
The actual tests are provided in the following commit
[WIP] unittests: Provide unittests for net_dev_dummy

* tests if net_dev_dummy is correct and if driver part of net_dev is
  correct