board/cpu: added SPI driver for stm32f0discovery
[SQUASH ME] finished 0.1 of spi driver
SPI: more fixes including DEBUG info
cpu: mini cleanup to spi driver
tests: added periph_spi test
[SQUASH ME] tests/periph_spi: rewrote test
[SQUASH ME] stm32f0: substantial fixes to SPI driver
[SQUASH ME] test: guard if no SPI device defined
[SQUASH ME] test: fixed according to updated GPIO interface
[SQUASH ME] cpu: removed slave mode
Merge pull request #1488 from haukepetersen/add_stm32f0_spi

board/cpu: added SPI driver for the stm32f0discovery