feat: process RC packets back from CRSF module as trainer input (#5724)

i.e. allow head tracker data to be relayed via backpack!

radio/src/gui/gui_common.cpp

@@ -1101,6 +1101,18 @@ bool isTrainerModeAvailable(int mode)
 #endif
   }
 
+  if (mode == TRAINER_MODE_CRSF) {
+
+#if !defined(CROSSFIRE)
+    return false;
+#else
+    if ((!IS_INTERNAL_MODULE_ENABLED() && !IS_EXTERNAL_MODULE_ENABLED()) ||
+         (!(isModuleELRS(INTERNAL_MODULE) && CRSF_ELRS_MIN_VER(INTERNAL_MODULE, 4, 0)) &&
+          !(isModuleELRS(EXTERNAL_MODULE) && CRSF_ELRS_MIN_VER(EXTERNAL_MODULE, 4, 0))))
+      return false;
+#endif
+  }
+
   return true;
 }