core: `sched_switch()` switch if not on runqueue

Fixes #1935.

`sched_switch()` should not only switch if the other priority is higher,
but also if the current thread was moved from the runqueue.
Don't evaluate inISR() if not needed
doc: extend i2c description for pull-up
Merge pull request #1958 from Kijewski/sched_switch-on-run-queue

core: `sched_switch()` switch if not on runqueue
Merge pull request #2199 from PeterKietzmann/adapt_i2c_doc

doc: extend i2c description for pull-up
driver/i2c: additional docu for pull resistors
Merge pull request #2200 from haukepetersen/fix_i2c_doc

driver/i2c: additional docu for pull resistors
driver/timer: adjusted doxygen for speed value
Merge pull request #2202 from haukepetersen/fix_driver_timerdoc

driver/timer: adjusted doxygen for speed value
board/mbed_lpc1768: added timer definitions
cpu/lpc1768: added basic timer implementation
Merge pull request #2168 from haukepetersen/add_lpc1768_timer

cpu/lpc1768: added basic timer implementation