Add MSP override mode

The MSP override mode allows for use of MSP togehter with
another RX feature like SBUS. When enabling the MSP override
mode all channels from the `msp_override_channels` bitmask
will be overwritten by data comming from MSP instead of the
main RX.

src/main/fc/rc_modes.h

@@ -74,6 +74,7 @@ typedef enum {
     BOXACROTRAINER,
     BOXVTXCONTROLDISABLE,
     BOXLAUNCHCONTROL,
+    BOXMSPOVERRIDE,
     CHECKBOX_ITEM_COUNT
 } boxId_e;