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/myir/mys_6ulx/Makefile b/board/myir/mys_6ulx/Makefile
index 3c63e43..9b76595 100644
--- a/board/myir/mys_6ulx/Makefile
+++ b/board/myir/mys_6ulx/Makefile
@@ -1,4 +1,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 
 obj-y  := mys_6ulx.o
-obj-$(CONFIG_SPL_BUILD) += spl.o
+obj-$(CONFIG_XPL_BUILD) += spl.o