Multi-Volume. Select Media for LVGL (#21344)

Marlin/Configuration_adv.h

@@ -1478,6 +1478,15 @@
   // Enable if SD detect is rendered useless (e.g., by using an SD extender)
   //#define NO_SD_DETECT
 
+  // Multiple volume support - EXPERIMENTAL.
+  //#define MULTI_VOLUME
+  #if ENABLED(MULTI_VOLUME)
+    #define VOLUME_SD_ONBOARD
+    #define VOLUME_USB_FLASH_DRIVE
+    #define DEFAULT_VOLUME SD_ONBOARD
+    #define DEFAULT_SHARED_VOLUME USB_FLASH_DRIVE
+  #endif
+
 #endif // SDSUPPORT
 
 /**