board: Use CONFIG_XPL_BUILD instead of CONFIG_SPL_BUILD

Use the new symbol to refer to any 'SPL' build, including TPL and VPL

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/liebherr/mccmon6/Makefile b/board/liebherr/mccmon6/Makefile
index 3c9786c..d5fdc20 100644
--- a/board/liebherr/mccmon6/Makefile
+++ b/board/liebherr/mccmon6/Makefile
@@ -2,7 +2,7 @@
 #
 # (C) Copyright 2016-2017
 # Lukasz Majewski, DENX Software Engineering, lukma@denx.de
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y  := spl.o
 else
 obj-y  := mccmon6.o