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/compulab/cl-som-imx7/Makefile b/board/compulab/cl-som-imx7/Makefile
index 8f0e068..35cf2c1 100644
--- a/board/compulab/cl-som-imx7/Makefile
+++ b/board/compulab/cl-som-imx7/Makefile
@@ -10,7 +10,7 @@
 
 obj-y := mux.o common.o
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y  += spl.o
 else
 obj-y  += cl-som-imx7.o