initial import of stm32f4discovery spi low-level driver
Merge pull request #1464 from PeterKietzmann/stm32f4_spi_driver

Initial Stm32f4 spi driver
Initialize addresses from CPU ID
auto_init: Put net_if auto-initialization into its own function
Merge pull request #837 from authmillenon/auto-init-addresses

auto_init: Initialize link-layer addresses from CPU ID
reverted forcing travis to install `gcc-arm-none-eabi=4-*`
Merge pull request #1742 from BytesGalore/revert_workaround_for_gcc_arm_none

Travis: reverted forcing travis to install `gcc-arm-none-eabi=4-*`
travis: remove comment obsoleted by #1742
Merge pull request #1743 from N8Fear/travis

travis: remove comment obsoleted by #1742
stm32f1: implement 32bit hwtimer by cascading two timers