AP_HAL_Linux: allow disco to use RCInput_UDP

This allows to use any device to send RCInput to Disco via network.
Particularly this is useful when paired to additional changes on
SkyController 2 so it can be used to control Ardupilot.

This was tested with https://github.com/lucasdemarchi/dema-rc running
on SkyController 2.

libraries/AP_HAL_Linux/HAL_Linux_Class.cpp

@@ -150,7 +150,7 @@ static RCInput_ZYNQ rcinDriver;
 #elif CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_BEBOP
 static RCInput_UDP  rcinDriver;
 #elif CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_DISCO
-static RCInput_Multi rcinDriver{2, new RCInput_SBUS, new RCInput_115200("/dev/uart-sumd")};
+static RCInput_Multi rcinDriver{3, new RCInput_SBUS, new RCInput_115200("/dev/uart-sumd"), new RCInput_UDP()};
 #elif CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_AERO
 static RCInput_SoloLink rcinDriver;
 #elif CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_NAVIO2 || \