GPS mode check. rtl_yaw_final option. YAW_RESETTOARMEDYAW overridable by user

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Randy
I've set the default to be, on rtl loitering above home, change yaw back to that when the quad was armed.  I think personally that is sensible and better than "no change" but you may think otherwise.  Changeable in config.h.

I obviously have modified the eeprom for the new parameter - but as far as I could see the enum numbers wouldn't change so I haven't incremented the version number.

ALSO: added GPS check before switching mode - with minor refactor.

Thanks
Gareth