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/msc/sm2s_imx8mp/Makefile b/board/msc/sm2s_imx8mp/Makefile
index 7908a0c..48dc885 100644
--- a/board/msc/sm2s_imx8mp/Makefile
+++ b/board/msc/sm2s_imx8mp/Makefile
@@ -4,7 +4,7 @@
 # SPDX-License-Identifier:      GPL-2.0
 #
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y += spl.o lpddr4_timing.o
 else
 obj-y += sm2s_imx8mp.o