stm32f1: implement RTT driver
Merge pull request #1613 from thomaseichinger/stm32f1_rtt

stm32f1:iot-lab_M3: implement RTT driver
sixlowpan: fix ipv6_addr_init_prefix
Merge pull request #1781 from authmillenon/sixlowpan-fix-prefix-init

sixlowpan: fix ipv6_addr_init_prefix
drivers: added low-level I2C driver interface
cpu/stm32f1: added low-level I2C driver
board/iot-lab_M3: added I2C device definitions
board/iot-lab_M3: added features
tests: added i2c driver test