boards: initial import for HiKoB Agilefox

This PR adds support for the HiKoB Fox board. It is very similar to the iot-lab_M3 board, only uses
an other SPI device to communicate with the at86rf231 radio device and pin mapping is different.
Come on it reads for itselfs:
For GPIO port B Pin 13 and 15 configurations in the high register are reseted and then set to alternate function output open-drain with max frequency of 10 MHz. 
:wink: