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/wandboard/Makefile b/board/wandboard/Makefile
index c3d8053..6e5bcc1 100644
--- a/board/wandboard/Makefile
+++ b/board/wandboard/Makefile
@@ -3,4 +3,4 @@
 # (C) Copyright 2013 Freescale Semiconductor, Inc.
 
 obj-y  := wandboard.o
-obj-$(CONFIG_SPL_BUILD) += spl.o
+obj-$(CONFIG_XPL_BUILD) += spl.o