board/iot-lab_M3: cleaned up SPI in periph_conf.h
cpu/stm32f1: cleanup in SPI driver

- made config more versatile
- added pin configuration to spi_init()
board/iot-lab_M3: cleanup of AT86RF231 driver

- removed board specific code
- added interface definitions to board.h
drivers/at86rf231: rework and clean-up

- moved all board specific code to low-level driver calls
- moved some functions from the board dir into at86rf231.c
- added doxygen headers