Add a basic serial_echo example, with a silly Easter Egg

examples/serial_echo/Makefile

@@ -0,0 +1,3 @@
+# Simple makefile for simple example
+PROGRAM=simple
+include ../../common.mk

examples/serial_echo/serial_echo.c

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+/* Extremely simple example that just reads from stdin and echoes back on stdout
+ *
+ * Has an easter egg, which is if you type "QUACK" it'll quack 3 times back at you.
+ *
+ * This example code is in the public domain
+ */
+#include "espressif/esp_common.h"
+#include <esp/uart.h>
+#include <string.h>
+
+void user_init(void)
+{
+    printf("SDK version:%s\n", sdk_system_get_sdk_version());
+    printf("Going into echo mode...\n");
+
+    /* By default stdout is line-buffered, so you only see output
+       after a newline is sent. This is helpful in a multithreaded
+       environment so output doesn't get chopped up within a line.
+
+       Here we want to see the echo immediately, so disable buffering.
+    */
+    setbuf(stdout, NULL);
+
+    while(1) {
+        int c = getchar();
+        if(c != EOF)
+            putchar(c);
+
+        /* Easter egg: check for a quack! */
+        static int quack;
+        if(c == "QUACK"[quack]) {
+            quack++;
+            if(quack == strlen("QUACK")) {
+                printf("\nQUACK\nQUACK\n");
+                quack = 0;
+            }
+        } else {
+            quack = 0;
+        }
+    }
+}