tests: fix netdev test
Merge pull request #1761 from authmillenon/netdev-fix

tests: fix netdev test
cpu/stm32f0: removed leftovers from slave mode
Merge pull request #1713 from haukepetersen/fix_stm32f0_spi

cpu/stm32f0: removed leftovers from slave mode
unittests: add test network device

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

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