Automatic disarm safety feature

Added automatic disarm after 5sec when "Don't spin the motors when armed" is enabled.
Users that don't have buzzer to warn when board is armed and use don't spin motors feature can forget to disarm the board. For example after landing they go to copter and then accidentally move throttle up when trying to remove flight battery.
I'd put that near failsafe-related stuff.
or at least near arming stuff.