sys/vtimer: spin on short duration in vtimer_sleep

closes #571
board: Initial import of stm32f4discovery
cpu: Initial import of stm32f4
cpu: Initial import of cortex-m4_common
test/examples: blacklisted stm32f4discovery
Merge pull request #1191 from haukepetersen/board_stm32f4discovery

board: Initial import of stm32f4discovery board
Merge pull request #1407 from LudwigOrtmann/issue-571

sys/vtimer: spin on short duration in vtimer_sleep
cpu: initial import of stm32f1
boards: initial import of iot-lab_M3
at86rf231: introduce variable to keep internal driver state

The at86rf231 radio transceiver provides one IRQ (TRX_END) signaling
end of transmission or reception but no way to distinguish between
these.