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.

boards/auav/x21/init/rc.board_mavlink

@@ -0,0 +1,7 @@
+#!/bin/sh
+#
+# AUAV/mRO x21 specific board MAVLink startup script.
+#------------------------------------------------------------------------------
+
+# Start MAVLink on the USB port
+mavlink start -d /dev/ttyACM0