sys/transceiver: make transceiver_pid volatile

- because it is
SQUASHME: clean out extern kernel_pid_t transceiver_pid redefinitions
cpu/cortex-mX_common: bugfix to arch_stack_init

- fixed cortex-[m0|m3|m4]
doc: fix flashrom doxygen
boards/cpu: fix some warnings
Merge pull request #1547 from haukepetersen/fix_cortex_threadarch

cpu/cortex-mX_common: bugfix to arch_stack_init
Merge pull request #1525 from LudwigOrtmann/morning-cleanup

doc, boards/cpu: cleanup
Merge pull request #1535 from LudwigOrtmann/transceiver_volatile

sys/transceiver: make transceiver_pid volatile