Initial import of i2c lowlevel driver for stm32f4
sys: net: Initial import of a general interface to a network protocol
Merge pull request #1448 from authmillenon/network-layer-interface

sys: net: Initial import of a general interface to a network protocol
Merge pull request #1859 from PeterKietzmann/add_stm32f4_i2c

cpu/stm32f4: added i2c driver implementation