[Modbus] Add KConfig for Modbus.

components/net/KConfig

@@ -122,4 +122,6 @@ config RT_USING_LWIP
 
 endmenu
 
+source "$RTT_DIR/components/net/freemodbus/KConfig"
+
 endmenu

components/net/freemodbus/KConfig

+menu "Modbus master and slave stack"
+
+config RT_USING_MODBUS
+	bool "Enable Modbus stack"
+	default n
+
+	if RT_USING_MODBUS
+		config RT_MODBUS_MASTER_RTU
+			bool "RTU master mode"
+			default n
+
+		config RT_MODBUS_SLAVE_RTU
+			bool "RTU slave mode"
+			default n
+	endif
+
+endmenu