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
@authmillenon I chose to keep `CPU_ID_LEN` in cpu-conf.h because to me it is more CPU than board specific.