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/sandbox/cpu/start.c b/arch/sandbox/cpu/start.c
index 4d3c806..81752ed 100644
--- a/arch/sandbox/cpu/start.c
+++ b/arch/sandbox/cpu/start.c
@@ -129,7 +129,7 @@
 }
 SANDBOX_CMDLINE_OPT_SHORT(help, 'h', 0, "Display help");
 
-#ifndef CONFIG_SPL_BUILD
+#ifndef CONFIG_XPL_BUILD
 int sandbox_main_loop_init(void)
 {
 	struct sandbox_state *state = state_get_current();