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/kontron/sl28/Makefile b/board/kontron/sl28/Makefile
index 084c11d..2cb4125 100644
--- a/board/kontron/sl28/Makefile
+++ b/board/kontron/sl28/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier:	GPL-2.0+
 
-ifndef CONFIG_SPL_BUILD
+ifndef CONFIG_XPL_BUILD
 obj-y += sl28.o cmds.o
 endif
 
@@ -8,7 +8,7 @@
 
 obj-$(CONFIG_ARMV8_PSCI) += psci.o
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-y += spl.o
 obj-$(CONFIG_SPL_ATF) += spl_atf.o
 endif