auto-disarm implementation, slight rewrite of HFMan commit 931478054e70c...

...d6a51916ea9430f041f61b7f7ba

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.