Improve SBus code.  Fix SBus Failsafe detection, tested with FrSky X8R
and X4RSB.  Check for end-byte to improve robustness.

Less logic.  Better definition of frame structure eliminates the '-1's.
LedStrip failsafe warning now correctly checks that the feature is
enabled.
Update failsafe documentation.
Merge remote-tracking branch 'upstream/master'
Update docs for new OpenLog firmware location