Mavlink startup script per board

-moved rc.mavlink to the boards optional rc additions (now it's called rc.board_mavlink) to handle board specific mavlink needs (mavlink over usb, ethernet, additional streams, etc.)
-mavlink module will be responsible to usb defaults, therefore less args are needed to be passed to mavlink module if one wants to use mavlink over usb.
-the way to check if connection is usb is by it's designated variable and not by config mode.

ROMFS/px4fmu_common/init.d/rcS

@@ -396,9 +396,15 @@ else
 	fi
 
 	#
-	# Start mavlink streams that are not configurable (e.g. on USB).
+	# Optional board mavlink streams: rc.board_mavlink
 	#
-	sh /etc/init.d/rc.mavlink
+	set BOARD_RC_MAVLINK /etc/init.d/rc.board_mavlink
+	if [ -f $BOARD_RC_MAVLINK ]
+	then
+		echo "Board extras: ${BOARD_RC_MAVLINK}"
+		sh $BOARD_RC_MAVLINK
+	fi
+	unset BOARD_RC_MAVLINK
 
 	#
 	# Start UART/Serial device drivers.