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/google/imx8mq_phanbell/Makefile b/board/google/imx8mq_phanbell/Makefile
index d6427cf..ac6418e 100644
--- a/board/google/imx8mq_phanbell/Makefile
+++ b/board/google/imx8mq_phanbell/Makefile
@@ -5,7 +5,7 @@
 
 obj-y += imx8mq_phanbell.o
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y += spl.o
 obj-$(CONFIG_IMX8M_LPDDR4) += lpddr4_timing_1g.o
 endif