arch: 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/arch/mips/Makefile b/arch/mips/Makefile
index a007914..453c788 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -2,7 +2,7 @@
 
 head-y := arch/mips/cpu/start.o
 
-ifeq ($(CONFIG_SPL_BUILD),y)
+ifeq ($(CONFIG_XPL_BUILD),y)
 head-$(CONFIG_ARCH_JZ47XX) := arch/mips/mach-jz47xx/start.o
 head-$(CONFIG_SOC_MT7621) := arch/mips/mach-mtmips/mt7621/spl/start.o
 endif