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/phytec/common/Makefile b/board/phytec/common/Makefile
index 04469d0..cd78f76 100644
--- a/board/phytec/common/Makefile
+++ b/board/phytec/common/Makefile
@@ -2,7 +2,7 @@
 # Copyright (C) 2023 PHYTEC Messtechnik GmbH
 # Author: Teresa Remmet <t.remmet@phytec.de>
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 # necessary to create built-in.o
 obj- := __dummy__.o
 endif