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/dhelectronics/dh_imx6/Makefile b/board/dhelectronics/dh_imx6/Makefile
index 70ca30d..7102a47 100644
--- a/board/dhelectronics/dh_imx6/Makefile
+++ b/board/dhelectronics/dh_imx6/Makefile
@@ -2,7 +2,7 @@
 #
 # Copyright (C) 2017 Marek Vasut <marex@denx.de>
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y	:= dh_imx6_spl.o
 else
 obj-y	:= dh_imx6.o