cpu: added cortexm common
Introduced a cleaned-up cpu/core interface

- Included a collection of cpu-dependent headers in core/include/arch
- Extracted all interfaces that need to be implemented for a cpu
- Created a mapping between those interfaces and the old ones

Conflicts:
	core/include/lpm.h
core: added flag for disabling arch interface
core: added missing state to lpm_arch interface
drivers: fixes to the low-level GPIO driver

- fixed doxygen comments
- added GPIO_UNDEFINED value
- added return values to all functions
drivers: gpio periph driver fixes 2
cpu: added cortex-M4 defines to cortexm-common
cpu: fixed cpu includes for cortexm-common
cpu: fixed periph/Makefile for stm32f4
cpu: rewrote uart driver for stm32f4