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 commit sets the default baud rate to this value and adapt
the `make term` target accordingly.
iot-lab_M3: don't overwrite a custom baud rate
iot-lab_M3: complement stdio documentation
Merge pull request #1940 from OlegHahm/iotlab_baudrate

iot-lab_M3: set baud rate to 500 kbaud