simulator: Rework dummy simulator code to user timer_irq / serial_irq

Change the simulator to use the generic timer_irq.c and serial_irq.c
code for (dummy) timer and io handling.  This is just to make the code
a better example for other developers (most micro-controllers will use
the timer_irq.c and serial_irq.c code).

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/simulator/serial.c

@@ -0,0 +1,43 @@
+// Example code for interacting with serial_irq.c
+//
+// Copyright (C) 2018  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include <fcntl.h> // fcntl
+#include <unistd.h> // STDIN_FILENO
+#include "board/serial_irq.h" // serial_get_tx_byte
+#include "sched.h" // DECL_INIT
+
+void
+serial_init(void)
+{
+    // Make stdin/stdout non-blocking
+    fcntl(STDIN_FILENO, F_SETFL, fcntl(STDIN_FILENO, F_GETFL, 0) | O_NONBLOCK);
+    fcntl(STDOUT_FILENO, F_SETFL, fcntl(STDOUT_FILENO, F_GETFL, 0) | O_NONBLOCK);
+}
+DECL_INIT(serial_init);
+
+static void
+do_uart(void)
+{
+    for (;;) {
+        uint8_t data;
+        int ret = serial_get_tx_byte(&data);
+        if (ret)
+            break;
+        else
+            write(STDOUT_FILENO, &data, sizeof(data));
+
+        // XXX - Normally the code would check if input data is
+        // available and call serial_rx_byte()
+    }
+}
+
+void
+serial_enable_tx_irq(void)
+{
+    // Normally this would enable the hardware irq, but we just call
+    // do_uart() directly in this demo code.
+    do_uart();
+}