ethernet: add iperf example to test real bandwidth

1. Add command for Ethernet based on console component.
2. Make cmd_system and iperf a component that can be referenced by other examples
3. Add "version" command to cmd_system.c
4. Clean up the README.md in all ethernet examples[TW#26525]

examples/system/console/main/cmd_decl.h

@@ -8,8 +8,13 @@
 */
 #pragma once
 
-// Register system functions
-void register_system();
+#ifdef __cplusplus
+extern "C" {
+#endif
 
-// Register WiFi functions
-void register_wifi();
+#include "cmd_system.h"
+#include "cmd_wifi.h"
+
+#ifdef __cplusplus
+}
+#endif