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/freescale/mx23evk/Makefile b/board/freescale/mx23evk/Makefile
index 6fe6992..5cc99ab 100644
--- a/board/freescale/mx23evk/Makefile
+++ b/board/freescale/mx23evk/Makefile
@@ -3,7 +3,7 @@
 # (C) Copyright 2000-2006
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 
-ifndef	CONFIG_SPL_BUILD
+ifndef	CONFIG_XPL_BUILD
 obj-y	:= mx23evk.o
 else
 obj-y	:= spl_boot.o