drivers: Initial import of SPI low-level driver IF
drivers: fixes to the SPI low-level interface
driver: updated SPI driver interface
[SQUASH ME] added fixed speed values
[SQUASH ME] removed UNDEFINED from device list
[SQUASH ME] fixed signature of spi_transfer_reg()
[SQUASH ME] added begin_transaction, fixed doc
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
Merge pull request #1311 from haukepetersen/add_driver_spi_if

drivers: Initial import of SPI low-level driver IF
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.