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/Makefile b/arch/sandbox/Makefile
index 2610794..f6cf859 100644
--- a/arch/sandbox/Makefile
+++ b/arch/sandbox/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0+
 
-head-y := arch/sandbox/cpu/start.o
-
+head-y := arch/sandbox/cpu/start.o arch/sandbox/cpu/os.o
+head-$(CONFIG_SANDBOX_SDL) += arch/sandbox/cpu/sdl.o
 libs-y += arch/sandbox/cpu/
 libs-y += arch/sandbox/lib/