fix various compiler warnings while building bs11_config

diff --git a/include/openbsc/rs232.h b/include/openbsc/rs232.h
new file mode 100644
index 0000000..deb0c6b
--- /dev/null
+++ b/include/openbsc/rs232.h
@@ -0,0 +1,8 @@
+#ifndef _RS232_H
+#define _RS232_H
+
+int rs232_setup(const char *serial_port, unsigned int delay_ms);
+
+int handle_serial_msg(struct msgb *msg);
+
+#endif /* _RS232_H */