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/cpu/Makefile b/arch/x86/cpu/Makefile
index 16e67e3..c21d6d6 100644
--- a/arch/x86/cpu/Makefile
+++ b/arch/x86/cpu/Makefile
@@ -15,7 +15,7 @@
 ifndef CONFIG_SPL
 extra-y	= start.o
 else
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 extra-y	= start_from_tpl.o
 else
 extra-y	= start_from_spl.o