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/variscite/imx93_var_som/Makefile b/board/variscite/imx93_var_som/Makefile
index b638839..8b160f1 100644
--- a/board/variscite/imx93_var_som/Makefile
+++ b/board/variscite/imx93_var_som/Makefile
@@ -8,7 +8,7 @@
 obj-y += imx93_var_som.o
 obj-$(CONFIG_TARGET_IMX93_VAR_SOM) += ../common/imx9_eeprom.o
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y += spl.o
 obj-$(CONFIG_TARGET_IMX93_VAR_SOM) += lpddr4x_timing.o
 else