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
at86rf231: use radio_packet_length_t
cc110x: use radio_packet_length_t
cc110x_ng: use radio_packet_length_t
cc2420: use radio_packet_length_t
Merge pull request #1375 from OlegHahm/radio_packet_length

Radio packet length
examples: use DEVELHELP per default
doc: add detailed scheduler documentation
Merge pull request #1530 from LudwigOrtmann/examples-develhelp

examples: use DEVELHELP per default
Merge pull request #1441 from LudwigOrtmann/scheduler_documentation

doc: add detailed scheduler documentation
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