board: cpu: initial import of iot-lab_M3

This is an initial port for the iot-lab_M3 board and stm32f1 cpu.
This is a rewrite so it is implemented without the use of ST's peripheral library.

Done:
- [x] context switching
- [x] timers
- [x] gpio
- [x] uart
- [x] stdio
- [x] spi
- [x] at86rf231
The mechanism is different now: You need to implement the function
```C
void cpuid_get(void *id);
```
as defined in `drivers/include/periph/cpuid.c` somewhere in your CPU implementation now (for native I did this in `periph_cpu.c` i.e.). The length of the ID is determined by the macro `CPUID_ID_LEN` in your board's `periph_conf.h`