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
core: sched_switch only switch for higher priority

sched_switch() is called by some library functions when a call unblocks
another thread. Then it needs to be tested if the current thread should
be preempted for the newly runnable thread.

A non-volutarily yield should only happen if the unblocked thread has a
_higher_ priority than the current thread. The current implementation,
which tests if the other thread has the same or a higher priority, does
not fit the documentation.
Merge pull request #1693 from Kijewski/sched_switch-only-if-higher

core: sched_switch only switch for higher priority
Merge pull request #1488 from haukepetersen/add_stm32f0_spi

board/cpu: added SPI driver for the stm32f0discovery