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/CMakeLists.txt

@@ -143,6 +143,7 @@ list(APPEND OPTIONAL_BOARD_RC
 	rc.board_defaults
 	rc.board_sensors
 	rc.board_extras
+	rc.board_mavlink
 )
 
 foreach(board_rc_file ${OPTIONAL_BOARD_RC})