Add identity when registering new device

Use nRF24 MAC address when a new KNoT device is being registered. The
id may be used by client apps to map known devices or allow to identify
repeated registration attempt of the same source.
diff --git a/src/knot_thing_protocol.c b/src/knot_thing_protocol.c
index a4b7674..31f996d 100644
--- a/src/knot_thing_protocol.c
+++ b/src/knot_thing_protocol.c
@@ -194,10 +194,12 @@
 
 	len = MIN(sizeof(msg.reg.devName), strlen(config.name));
 	msg.hdr.type = KNOT_MSG_REGISTER_REQ;
+	msg.reg.id = config.mac.address.uint64; /* Maps id to nRF24 MAC */
 	strncpy(msg.reg.devName, config.name, len);
-	msg.hdr.payload_len = len;
+	msg.hdr.payload_len = len + sizeof(msg.reg.id);
 
-	if (hal_comm_write(cli_sock, &(msg.buffer), sizeof(msg.hdr) + len) < 0)
+	if (hal_comm_write(cli_sock, &(msg.buffer),
+			   sizeof(msg.hdr) + msg.hdr.payload_len) < 0)
 		return -1;
 
 	return 0;