Copter: only allow CLI within 20s of startup and when motors not armed
Rover: change CLI timeout to 20s
Plane: change CLI timeout to 20s
GCS_MAVLink: added comm_is_idle() function

this will make CLI detection more reliable
Copter: only enter CLI is link is idle when 3 enters are hit

this will prevent binary contents of RADIO packets being interpreted
as CLI enter line-feeds
Plane: only enter CLI is link is idle when 3 enters are hit
    
this will prevent binary contents of RADIO packets being interpreted
as CLI enter line-feeds
Rover: only enter CLI is link is idle when 3 enters are hit
    
this will prevent binary contents of RADIO packets being interpreted
as CLI enter line-feeds