drivers: fixes to the low-level GPIO driver

- fixed doxygen comments
- added GPIO_UNDEFINED value
- added return values to all functions
Merge pull request #971 from haukepetersen/periph_gpio_fix

drivers: fixes to the low-level GPIO driver
cpu: Initial import of cortexm_common files

- fixes to cortexm-common
- Added ARM CMSIS headers to cortexm_common
- thread_exit fixes to cortexm-common
- cortexm-common adjusted documentation
cpu: Initial import of SAM3X8E

- fixed spelling in uart driver
- marked cortexm-common a phony target
- fixed Makefiles
- added reboot interface implementation
- fixed documentation
board: Initial import of arduino-due

- switched to flashing using the programming port
- fixed Makefile.include
- fixes some documentation