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/px4/fmu-v4/init/rc.board_extras

@@ -3,13 +3,6 @@
 # PX4 FMUv4 specific board extras init
 #------------------------------------------------------------------------------
 
-
-# Pixracer: start MAVLink on Wifi (ESP8266 port). Except for the TealOne airframe.
-if ! param compare SYS_AUTOSTART 4250
-then
-	mavlink start -r 20000 -b 921600 -d /dev/ttyS0
-fi
-
 # Run FrSky Telemetry on Pixracer on the FrSky port if not enabled already
 if param compare TEL_FRSKY_CONFIG 0
 then

boards/px4/fmu-v4/init/rc.board_mavlink

@@ -0,0 +1,13 @@
+#!/bin/sh
+#
+# PX4 FMUv4 specific board MAVLink startup script.
+#------------------------------------------------------------------------------
+
+# Start MAVLink on the USB port
+mavlink start -d /dev/ttyACM0
+
+# Pixracer: start MAVLink on Wifi (ESP8266 port). Except for the TealOne airframe.
+if ! param compare SYS_AUTOSTART 4250
+then
+	mavlink start -r 20000 -b 921600 -d /dev/ttyS0
+fi