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/components/cmd_system/cmd_system.h

@@ -0,0 +1,20 @@
+/* Console example — various system commands
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Register system functions
+void register_system();
+
+#ifdef __cplusplus
+}
+#endif