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/sandbox/cpu/Makefile b/arch/sandbox/cpu/Makefile
index 7c5c526..bfcdc33 100644
--- a/arch/sandbox/cpu/Makefile
+++ b/arch/sandbox/cpu/Makefile
@@ -8,7 +8,7 @@
 obj-y	:= cache.o cpu.o state.o
 extra-y	:= start.o os.o
 extra-$(CONFIG_SANDBOX_SDL)    += sdl.o
-obj-$(CONFIG_SPL_BUILD)	+= spl.o
+obj-$(CONFIG_XPL_BUILD)	+= spl.o
 obj-$(CONFIG_ETH_SANDBOX_RAW)	+= eth-raw-os.o
 
 # os.c is build in the system environment, so needs standard includes