🔧 Added MOTHERBOARD error text

Author: thinkyhead

Marlin/src/inc/SanityCheck.h

@@ -88,7 +88,7 @@
  * Warnings for old configurations
  */
 #ifndef MOTHERBOARD
-  #error "MOTHERBOARD is required."
+  #error "MOTHERBOARD is required. You must '#define MOTHERBOARD BOARD_MYNAME' (not just '#define BOARD_MYNAME')."
 #elif !defined(X_BED_SIZE) || !defined(Y_BED_SIZE)
   #error "X_BED_SIZE and Y_BED_SIZE are now required!"
 #elif WATCH_TEMP_PERIOD > 500