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/mach-jz47xx/Makefile b/arch/mips/mach-jz47xx/Makefile
index dbb8229..5621a09 100644
--- a/arch/mips/mach-jz47xx/Makefile
+++ b/arch/mips/mach-jz47xx/Makefile
@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0+
 
-extra-$(CONFIG_SPL_BUILD)	:= start.o
+extra-$(CONFIG_XPL_BUILD)	:= start.o
 
 obj-$(CONFIG_SOC_JZ4780)	+= jz4780/