mavlink USB auto start/stop on boards with VBUS

- no longer start sercon or mavlink usb by default
 - on USB connection (VBUS) monitor serial USB at low rate and start Mavlink if there's a HEARTBEAT or nshterm on 3 consecutive carriage returns
 - the mavlink USB instance is automatically stopped and serdis executed if USB is disconnected
 - skipping Mavlink USB (and sercon) saves a considerable amount of memory on older boards

boards/cubepilot/cubeorange/init/rc.board_mavlink

@@ -3,9 +3,6 @@
 # Board specific MAVLink startup script.
 #------------------------------------------------------------------------------
 
-# Start MAVLink on the USB port
-mavlink start -d /dev/ttyACM0
-
 # Start ADS-B receiver mavlink connection if console not present
 if [ ! -e /dev/console ]
 then

boards/cubepilot/cubeyellow/init/rc.board_mavlink

@@ -1,7 +0,0 @@
-#!/bin/sh
-#
-# Board specific MAVLink startup script.
-#------------------------------------------------------------------------------
-
-# Start MAVLink on the USB port
-mavlink start -d /dev/ttyACM0