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/advantech/imx8mp_rsb3720a1/Makefile b/board/advantech/imx8mp_rsb3720a1/Makefile
index eb6b18b..39fb0d4 100644
--- a/board/advantech/imx8mp_rsb3720a1/Makefile
+++ b/board/advantech/imx8mp_rsb3720a1/Makefile
@@ -8,7 +8,7 @@
 ifdef CONFIG_TARGET_IMX8MP_RSB3720A1_6G
 obj-y += imx8mp_rsb3720a1.o
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y += spl.o
 obj-$(CONFIG_IMX8M_LPDDR4) += lpddr4_timing_rsb3720a1_6G.o
 endif
@@ -17,7 +17,7 @@
 ifdef CONFIG_TARGET_IMX8MP_RSB3720A1_4G
 obj-y += imx8mp_rsb3720a1.o
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y += spl.o
 obj-$(CONFIG_IMX8M_LPDDR4) += lpddr4_timing_rsb3720a1_4G.o
 endif