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/siemens/rut/Makefile b/board/siemens/rut/Makefile
index aae5364..ac59c5b 100644
--- a/board/siemens/rut/Makefile
+++ b/board/siemens/rut/Makefile
@@ -9,13 +9,13 @@
 # u-boot:/board/ti/am335x/Makefile
 # Copyright (C) 2011 Texas Instruments Incorporated - https://www.ti.com/
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y	:= mux.o
 endif
 
 obj-y	+= board.o
 obj-y += ../common/board_am335x.o
 obj-y += ../common/eeprom.o
-ifndef CONFIG_SPL_BUILD
+ifndef CONFIG_XPL_BUILD
 obj-y += ../common/factoryset.o
 endif