Initial import of low-level timer driver interface
Initial import of low-level UART driver interface
Fixed spelling
drivers: remodeled low-level uart driver interface

- added blocking mode of operation
- added UNDEFINED device for compatibility
- changed baudrate type to uint32_t
cpu: added cortexm common
core/cpu/board: moved F_CPU define to board
core: changed e|dINT calls to en|disableIRQ
core: changed e|dINT to en|disableIRQ in hwtimer.c
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: adjustments to low-level timer driver IF
cpu: fixes to cortexm-common
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
board: Initial import of arduino-due
cpu: Initial import of SAM3X8E
cpu: Added ARM CMSIS headers to cortexm_common
cpu: thread_exit fixes to cortexm-common
cpu: sam3x8e fixed spelling in uart driver