tests/periph_spi: let build fail without SPI
make: introduce and use periph_spi feature
documentation: added description for packet queue
core: refactor check for valid pid

move into header and remove redundant condition
core: add doxygen header for kernel_types
core/msg: add DEVELHELP checks for valid pid
Merge pull request #1810 from OlegHahm/pktqueue_doxygen_group_brief

documentation: added description for packet queue
Merge pull request #1794 from LudwigOrtmann/valid_pid

core: refactor check for valid pid
Merge pull request #1809 from LudwigOrtmann/Makefile.features-SPI

make: periph_spi feature
drivers: added low-level I2C driver interface
cpu/stm32f1: added low-level I2C driver
board/iot-lab_M3: added I2C device definitions
board/iot-lab_M3: added features
tests: added i2c driver test