sandbox: fix compiler warning

Add back return statement to fix compiler warning about control flow
reaching end of non void function that was introduced with:

	e05e5de7 arm: move C runtime setup code in crt0.S

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/start.c

@@ -122,4 +122,7 @@ int main(int argc, char *argv[])
 	 * never return.
 	 */
 	board_init_f(0);
+
+	/* NOTREACHED - board_init_f() does not return */
+	return 0;
 }