boards/iot-lab_M3: change to openocd's new ftdi interface

Openocd communication was done over a deprecated interface definition,
namely ft2232. Using the new ftdi interface speeds up flashing and
doesn't throw warnings anymore.
Merge pull request #1779 from thomaseichinger/stm32f1-flashing

boards/iot-lab_M3: change to openocd's new ftdi interface