stm32f1:iot-lab_M3: implement RTT driver

Just thinking loud: as we have only a 16-bit hwtimer on this platform, would it make sense, to set the hwtimer to 1MHz (which leads to period of ~64ms), and program the RTT to run with something around 20 Hz (period of 50ms)? Or should we first think of how to integrate the RTT with the vtimer and then come back to this question?