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/nvidia/beaver/Makefile b/board/nvidia/beaver/Makefile
index 5e9e708..d56c715 100644
--- a/board/nvidia/beaver/Makefile
+++ b/board/nvidia/beaver/Makefile
@@ -2,6 +2,6 @@
 #
 # Copyright (c) 2010-2013, NVIDIA CORPORATION.  All rights reserved.
 
-obj-$(CONFIG_SPL_BUILD) += beaver-spl.o
+obj-$(CONFIG_XPL_BUILD) += beaver-spl.o
 
 obj-y	= ../cardhu/cardhu.o