cpu/stm32f4: added i2c driver implementation

This PR implements the I2C low-level driver interface for stm32f4. It will be tested in more detail in another PR where the I2C-srf02 sensor is adapted to the general interface.
This should be `pin_scl`, right?
And sure you mean a pre-processor `#if` here? The value of `port_scl` is not constant on compile time... What happens if you have two I2C devices, one has port_scl = 3 and the other one port_scl = 12? -> use `if(..) {` instead.