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/include/asm/main.h b/arch/sandbox/include/asm/main.h
new file mode 100644
index 0000000..7a2f0d3
--- /dev/null
+++ b/arch/sandbox/include/asm/main.h
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (c) 2022 Google, Inc.
+ * Written by Andrew Scull <ascull@google.com>
+ */
+
+#ifndef __ASM_SANDBOX_MAIN_H
+#define __ASM_SANDBOX_MAIN_H
+
+/**
+ * sandbox_main() - main entrypoint for sandbox
+ *
+ * @argc:	the number of arguments passed to the program
+ * @argv:	array of argc+1 pointers, of which the last one is null
+ */
+int sandbox_main(int argc, char *argv[]);
+
+#endif /* __ASM_SANDBOX_MAIN_H */