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/x86/Makefile b/arch/x86/Makefile
index f1afc74..3c8c541 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -10,7 +10,7 @@
 ifndef CONFIG_SPL
 head-y := arch/x86/cpu/start.o
 else
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 head-y	= arch/x86/cpu/start_from_tpl.o
 else
 head-y	= arch/x86/cpu/start_from_spl.o