Add FileHandle_sigio example

Author: mprse

APIs_Platform/FileHandle_sigio/README.md

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+## FileHandle sigio example
+
+The example monitors a serial console device, and every time it outputs a character, send it to the console back and toggle LED2.
+
+1. Flash the board, and ensure the target's USB is plugged into the PC.
+2. Open serial console (select associated COM port, transmission speed: 19200 bps).
+3. Reset the target.
+4. Every time that device outputs a character, the character should appear in the console and LED2 should be toggled LED2.
+
+**Note:** You can check the associated serial port using `mbedls` command.

APIs_Platform/FileHandle_sigio/main.cpp

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+// Main thread 
+#include "mbed.h"
+
+static DigitalOut led1(LED1);
+static DigitalOut led2(LED2);
+
+static BufferedSerial device(STDIO_UART_TX, STDIO_UART_RX);
+
+static void callback_ex()
+{
+    // always read until data is exhausted - we may not get another
+    // sigio otherwise
+    while (1) {
+        char c;
+        if (device.read(&c, 1) != 1) {
+            break;
+        }
+        putchar(c);
+        led2 = !led2;
+    }
+}
+
+int main()
+{
+    // BufferedSerial-specific method - all others are from FileHandle base class
+    device.set_baud(19200);
+
+    // Ensure that device.read() returns -EAGAIN when out of data
+    device.set_blocking(false);
+
+    // sigio callback is deferred to event queue, as we cannot in general
+    // perform read() calls directly from the sigio() callback.
+    device.sigio(mbed_event_queue()->event(callback_ex));
+
+    while (1) {
+        led1 = !led1;
+        ThisThread::sleep_for(500);
+    }
+}