sandbox: Use 'extras' to specify 'head' files

At present sandbox has a start.o in the 'start' target but also includes
it in the normal target list. This is not how this is normally handled. It
is needed because sandbox does not include the u-boot-init variable in its
link rule.

Update the rule and move start.o from the normal target list to the
'extras' list.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/Makefile b/arch/sandbox/cpu/Makefile
index 8fe6818..bac9644 100644
--- a/arch/sandbox/cpu/Makefile
+++ b/arch/sandbox/cpu/Makefile
@@ -5,10 +5,11 @@
 # (C) Copyright 2000-2003
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 
-obj-y	:= cpu.o os.o start.o state.o
+obj-y	:= cpu.o state.o
+extra-y	:= start.o os.o
+extra-$(CONFIG_SANDBOX_SDL)	+= sdl.o
 obj-$(CONFIG_SPL_BUILD)	+= spl.o
 obj-$(CONFIG_ETH_SANDBOX_RAW)	+= eth-raw-os.o
-obj-$(CONFIG_SANDBOX_SDL)	+= sdl.o
 
 # os.c is build in the system environment, so needs standard includes
 # CFLAGS_REMOVE_os.o cannot be used to drop header include path