removed duplicate inISR prototype
simplified sched_switch

sched_switch can check ISR itself.
Make at86rf231_switch_to_rx() public to use with transceiver
Merge pull request #783 from authmillenon/fix-at86rf231-followup

drivers : Make at86rf231_switch_to_rx() public to use with transceiver
Merge pull request #713 from OlegHahm/sched_switch_simplification

core: move inISR check into sched_switch