Board stm32f0discovery

Imports support for the stm32f0discovery board an with it for the ARM Cortex-M0 platform.

The peripheral drivers (timer, uart, gpio and exti) are thoroughly tested, the lpm implementation is still missing.

~~This PR depends on:~~
- ~~#1288~~
- ~~#1289~~
[cpu/cortex-m0_common/thread_arch.c:134]: (warning) %u in format string (no. 1) requires 'unsigned int' but the argument type is 'int