Added auto_disarm_board capability for copters.  Ported from jaahaavi commit on baseflight.

Ported from jaahaavi commit on baseflight.  Added automatic disarm after 5 seconds (important safety feature) when feature MOTOR_STOP is enabled (has no effect on FIXED_WING configurations.)  Users that don't have a buzzer to warn when board is armed and use feature MOTOR_STOP can forget to disarm the board. For example after landing they pick up copter and then accidentally move throttle up when trying to remove flight battery. 

Configurable via CLI using 'set auto_disarm_board=x' where x is 0-60 seconds.  If zero, does not auto_disarm.  Defaults to 5 seconds.