Use config files to store whether SDMC is enabled or not

Before, it used to use whether the directory actually existed. As a result, .citra-emu/sdmc was never auto-created (something quite confusing to me until I read through the logs).

src/core/settings.h

@@ -24,6 +24,8 @@ struct Values {
     int pad_sdown_key;
     int pad_sleft_key;
     int pad_sright_key;
+
+    bool use_virtual_sd;
 } extern values;
 
 }