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

ROMFS/cannode/init.d/rcS

@@ -16,11 +16,6 @@ set +e
 #------------------------------------------------------------------------------
 set R /
 
-#
-# Start CDC/ACM serial driver.
-#
-sercon
-
 #
 # Print full system version.
 #

ROMFS/px4fmu_common/init.d/rcS

@@ -49,11 +49,6 @@ set STARTUP_TUNE 1
 set USE_IO no
 set VEHICLE_TYPE none
 
-#
-# Start CDC/ACM serial driver.
-#
-sercon
-
 #
 # Print full system version.
 #

ROMFS/px4fmu_test/init.d/rcS

@@ -13,14 +13,6 @@ then
 	led_control on -c blue
 fi
 
-if sercon
-then
-	echo "[i] USB interface connected"
-
-	# Try to get an USB console
-	nshterm /dev/ttyACM0 &
-fi
-
 #
 # Try to mount the microSD card.
 #