Add sanity-check for MOTHERBOARD

Marlin/src/inc/SanityCheck.h

@@ -54,7 +54,9 @@
 /**
  * Warnings for old configurations
  */
-#if !defined(X_BED_SIZE) || !defined(Y_BED_SIZE)
+#ifndef MOTHERBOARD
+  #error "MOTHERBOARD is required. Please update your configuration."
+#elif !defined(X_BED_SIZE) || !defined(Y_BED_SIZE)
   #error "X_BED_SIZE and Y_BED_SIZE are now required! Please update your configuration."
 #elif WATCH_TEMP_PERIOD > 500
   #error "WATCH_TEMP_PERIOD now uses seconds instead of milliseconds."