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/technexion/pico-imx6ul/spl.c b/board/technexion/pico-imx6ul/spl.c
index 67484e6..5b91868 100644
--- a/board/technexion/pico-imx6ul/spl.c
+++ b/board/technexion/pico-imx6ul/spl.c
@@ -18,7 +18,7 @@
 #include <linux/libfdt.h>
 #include <spl.h>
 
-#if defined(CONFIG_SPL_BUILD)
+#if defined(CONFIG_XPL_BUILD)
 
 #ifdef CONFIG_SPL_OS_BOOT
 int spl_start_uboot(void)