crypto: get rid of compiler warnings
core: only store the stack size for DEVELHELP

`tcp_t::stack_size` is only examined by the shell command `ps` and
`DEBUG_PRINT`. For the latter one only if `DEVELHELP` was enabled.

This PR guards the member `tcp_t::stack_size` in `#ifdef DEVELHELP`.
Only if DEVELHELP was activated its value get printed by `ps`.

Closes #1287.
core: thread_measure_stack_free() is only useful for DEVELHELP
tests: let license checker check headers, too
sys/transceiver: make transceiver_pid volatile

- because it is
SQUASHME: clean out extern kernel_pid_t transceiver_pid redefinitions
boards: fixup for pyterm renaming
Merge pull request #1543 from LudwigOrtmann/pyterm-fixup

boards: fixup for pyterm renaming
tests, native: fixup for cpuid

- adopt test name to new scheme
- make cppcheck happy / dont overwrite the target memory location twice
- fix license header
- don't redefine define if it already exists
- don't write more cpuid than is defined as size
doc: fix spelling in core/include
Merge pull request #1545 from LudwigOrtmann/spellingbee

doc: fix spelling in core/include
buildtest: retry failed builds once

On Travis CI often builds fail spuriously.

This PR lets `make buildtest` retry the build once.
tests: fix cppcheck findings

- bitarithm_timings: use more void, decrease range
- struct_tm_utility: add cppcheck-suppress invalidscanf
core/queue: queue -> priority_queue

Rename queue to priority queue, because that's what it is.
core/priority_queue: add dynamic initializers

- priority_queue_init
- priority_queue_node_init
Merge pull request #1527 from LudwigOrtmann/cppcheck-tests

tests: fix cppcheck findings
Merge pull request #1534 from LudwigOrtmann/queue-cleanup-two

core/queue: queue -> priority_queue && dynamic initializers
cpu/cortex-mX_common: bugfix to arch_stack_init

- fixed cortex-[m0|m3|m4]
Merge pull request #1529 from LudwigOrtmann/cpuid_fixup

tests, native: fixup for cpuid
doc: fix flashrom doxygen