iot-lab_M3: set baud rate to 500 kbaud

The IoT-Lab testbed requires a baud rate of 500 kbaud to use UART.
Hence, this PR sets the default baud rate to this value and adapt
the `make term` target accordingly.
I indent `#ifdef` blocks, because it is much easier to read. But true, the indentation of `(500000U)` is off.