x8: add echo server example

Author: facchinm

libraries/RPC/examples/PortentaX8_EchoServer/PortentaX8_EchoServer.ino

@@ -0,0 +1,41 @@
+#include "SerialRPC.h"
+
+/*
+ * This sketch demonstrates how to interact with the Portenta X8 Serial port (over USB)
+ * On the board, launch both 'proxy' and 'example' binaries (from https://github.com/bcmi-labs/portentax8-m4-proxy)
+ * The M4 provides the 'subtract' API (which will be invoked by 'example'
+ * It also provides a full duplex Serial-like interface that is proxies through the serial monitor
+ * Last but not leas, when you write 'echo' the corresponding function in 'example' will be triggered
+ */
+
+int subtract(int a, int b) {
+  return a-b;
+}
+
+void setup() {
+  // put your setup code here, to run once:
+  Serial.begin(115200);
+  RPC.bind("subtract", subtract);
+  delay(1000);
+}
+
+int i = 0;
+void loop() {
+
+  //RPC.print("hello");
+  //RPC.send("echo", "test");
+  //auto res = RPC.call("add", 5, 8).as<int>();
+  //RPC.send("echo", String(res).c_str());
+
+  String str = "";
+  while (Serial.available()) {
+    str += (char)Serial.read();
+  }
+  if (str != "") {
+    Serial.print(str);
+  }
+  if (str.startsWith("echo")) {
+    delay(100);
+    RPC.send("echo", "test");
+  }
+}