sandbox: Decouple program entry from sandbox init

Move the program's entry point to os.c, in preparation for a separate
fuzzing entry point to be added.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/start.c b/arch/sandbox/cpu/start.c
index 0f5a873..90a84e9 100644
--- a/arch/sandbox/cpu/start.c
+++ b/arch/sandbox/cpu/start.c
@@ -453,7 +453,7 @@
 	os_relaunch(os_argv);
 }
 
-int main(int argc, char *argv[])
+int sandbox_main(int argc, char *argv[])
 {
 	struct sandbox_state *state;
 	void * text_base;