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.
```c
#ifndef STDIO_BAUDRATE
#   define STDIO_BAUDRATE (500000U)
#endif
```