Fix continuous beeping when rate profile selection is used.
Ported from jaahaavi commit on baseflight.  Added automatic disarm after
5 seconds 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.