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/beacon/imx8mm/Makefile b/board/beacon/imx8mm/Makefile
index 7d3bd31..b33dc8b 100644
--- a/board/beacon/imx8mm/Makefile
+++ b/board/beacon/imx8mm/Makefile
@@ -7,7 +7,7 @@
 obj-y += imx8mm_beacon.o
 obj-y += ../../freescale/common/
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y += spl.o
 obj-y += lpddr4_timing.o
 endif