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/renesas/alt/Makefile b/board/renesas/alt/Makefile
index 5341869..f9c131f 100644
--- a/board/renesas/alt/Makefile
+++ b/board/renesas/alt/Makefile
@@ -6,7 +6,7 @@
 # SPDX-License-Identifier: GPL-2.0
 #
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y	:= alt_spl.o
 else
 obj-y	:= alt.o qos.o