arch: 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/arch/arm/mach-imx/imx9/Makefile b/arch/arm/mach-imx/imx9/Makefile
index e1b09ab..45a9105 100644
--- a/arch/arm/mach-imx/imx9/Makefile
+++ b/arch/arm/mach-imx/imx9/Makefile
@@ -5,6 +5,6 @@
 obj-y += lowlevel_init.o
 obj-y += soc.o clock.o clock_root.o trdc.o
 
-#ifndef CONFIG_SPL_BUILD
+#ifndef CONFIG_XPL_BUILD
 obj-y += imx_bootaux.o
 #endif