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
yupp, they can. The trick here is, that the Cortex-M saves a set of registers (r0-r3, pc, lr...) per hardware when entering an interrupt, so gcc does not have to take care of this...