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
@thomaseichinger : (i'm not a RIOT maintainer)
Could we keep the old names of these interrupt handlers (e.g SysTick_Handler instead of isr_systick, USART1_IRQHandler instead of isr_usart1, etc.). I know this will violate RIOT's coding convention but it can be compatible with other user's code.
For example, Cortex-M system exception handler names (SysTick_Handler, PendSV_Handler, etc.) are defined in CMSIS and should be common for all cortex mcus.
And old stm32f1 specific interrupt names should be kept to be compatible with the code (libraries, drivers, etc.) which are using ST's library. :-)