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/timer.c

@@ -0,0 +1,109 @@
+// Example code for running timers in a polling mode
+//
+// Copyright (C) 2018  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include <time.h> // struct timespec
+#include <unistd.h> // usleep
+#include "autoconf.h" // CONFIG_CLOCK_FREQ
+#include "board/irq.h" // irq_disable
+#include "board/misc.h" // timer_from_us
+#include "board/timer_irq.h" // timer_dispatch_many
+#include "command.h" // DECL_CONSTANT
+#include "sched.h" // DECL_INIT
+
+// Helper function that returns the system time as a 32bit counter
+static uint32_t
+get_system_time(void)
+{
+    struct timespec ts;
+    clock_gettime(CLOCK_MONOTONIC, &ts);
+    double t = (double)ts.tv_sec + (double)ts.tv_nsec * .000000001;
+    return (uint32_t)(t * CONFIG_CLOCK_FREQ);
+}
+
+
+/****************************************************************
+ * Timers
+ ****************************************************************/
+
+static uint32_t next_wake_time;
+
+// Return the current time (in absolute clock ticks).
+uint32_t
+timer_read_time(void)
+{
+    return get_system_time();
+}
+
+// Activate timer dispatch as soon as possible
+void
+timer_kick(void)
+{
+    next_wake_time = timer_read_time();
+}
+
+// Invoke timers - called below from irq_poll()
+static void
+do_timer_dispatch(void)
+{
+    next_wake_time = timer_dispatch_many();
+}
+
+void
+timer_init(void)
+{
+    timer_kick();
+}
+DECL_INIT(timer_init);
+
+
+/****************************************************************
+ * Interrupts
+ ****************************************************************/
+
+// Disable hardware interrupts
+void
+irq_disable(void)
+{
+}
+
+// Enable hardware interrupts
+void
+irq_enable(void)
+{
+}
+
+// Disable hardware interrupts in not already disabled
+irqstatus_t
+irq_save(void)
+{
+    return 0;
+}
+
+// Restore hardware interrupts to state from flag returned by irq_save()
+void
+irq_restore(irqstatus_t flag)
+{
+}
+
+// Atomically enable hardware interrupts and sleep processor until next irq
+void
+irq_wait(void)
+{
+    // XXX - sleep to prevent excessive cpu usage in simulator
+    usleep(1);
+
+    irq_poll();
+}
+
+// Check if an interrupt is active (used only on architectures that do
+// not have hardware interrupts)
+void
+irq_poll(void)
+{
+    uint32_t now = timer_read_time();
+    if (!timer_is_before(now, next_wake_time))
+        do_timer_dispatch();
+}