fix: Don't allow select MULTI_TRAINER if no MULTI (#2299)

* fix: Don't allow select MULTI_TRAINER if no MULTI
Fixes #1563 by ensuring Multi Trainer opt is not shown if no MPM
is enabled, as well as if MPM suport is not even compiled in.

* chore: Uppercase "OFF" or equiv for Trainer OFF

radio/src/gui/gui_common.cpp

@@ -882,7 +882,18 @@ bool isTrainerModeAvailable(int mode)
        mode == TRAINER_MODE_MASTER_CPPM_EXTERNAL_MODULE))
     return false;
 #endif
-  
+
+#if !defined(MULTIMODULE)
+  if (mode == TRAINER_MODE_MULTI) 
+    return false;
+#else
+  if (mode == TRAINER_MODE_MULTI &&
+      ((!IS_INTERNAL_MODULE_ENABLED() && !IS_EXTERNAL_MODULE_ENABLED()) ||
+       (!isModuleMultimodule(INTERNAL_MODULE) &&
+        !isModuleMultimodule(EXTERNAL_MODULE))))
+    return false;
+#endif
+
   return true;
 }