Prevent MB false-positive on bad board name

Marlin/src/core/boards.h

@@ -226,6 +226,6 @@
 #define BOARD_THE_BORG         1860   // THE-BORG (Power outputs: Hotend0, Hotend1, Bed, Fan)
 
 
-#define MB(board) (MOTHERBOARD==BOARD_##board)
+#define MB(board) (defined(BOARD_##board) && MOTHERBOARD==BOARD_##board)
 
 #endif // __BOARDS_H