#565 - set rc_override_active based on if any overrides are actually

sent by the GCS.  previously rc_override_active would be set to true
if any override packet arrived - this is not correct because the GCS
sends a packet full of zeros to turn overrides _off_.

Since override behavior is common to nearly all HALs.  I moved the
rc_override code into the base class (under the DRY sw pricipal ;-)).

This is a WIP so DO NOT MERGE for now (best to wait for the AC3.1 branch
anyways).  I need to test it with plane and copter.  There are also a
couple of other rc_overrideish edge conditions we should address...

The only thing I ask is please no big refactoring on these files while I
have this change sitting out there - to avoid merge hell once I can check
in a complete set of changes...