feat(driver/net): 实现Loopback网卡接口 (#845)

* 初步实现loopback设备

user/apps/test_lo/src/main.rs

@@ -0,0 +1,25 @@
+use std::net::UdpSocket;
+use std::str;
+
+fn main() -> std::io::Result<()> {
+    let socket = UdpSocket::bind("127.0.0.1:34254")?;
+    socket.connect("127.0.0.1:34254")?;
+
+    let msg = "Hello, loopback!";
+    socket.send(msg.as_bytes())?;
+
+    let mut buf = [0; 1024];
+    let (amt, _src) = socket.recv_from(&mut buf)?;
+
+    let received_msg = str::from_utf8(&buf[..amt]).expect("Could not read buffer as UTF-8");
+
+    println!("Sent: {}", msg);
+    println!("Received: {}", received_msg);
+
+    assert_eq!(
+        msg, received_msg,
+        "The sent and received messages do not match!"
+    );
+
+    Ok(())
+}