linux: Initial support for running Klipper in a Linux real-time process

Add support for compiling the Klipper micro-controller code as a
real-time process capable of running on standard Linux systems.

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

src/linux/watchdog.c

@@ -0,0 +1,36 @@
+// Support for Linux watchdog
+//
+// Copyright (C) 2017  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include <fcntl.h> // open
+#include <unistd.h> // write
+#include "internal.h" // report_errno
+#include "sched.h" // DECL_TASK
+
+static int watchdog_fd = -1;
+
+int
+watchdog_setup(void)
+{
+    int ret = open("/dev/watchdog", O_RDWR|O_CLOEXEC);
+    if (ret < 0) {
+        report_errno("watchdog open", ret);
+        return -1;
+    }
+    watchdog_fd = ret;
+    return set_non_blocking(watchdog_fd);
+}
+
+void
+watchdog_task(void)
+{
+    static struct timespec next_watchdog_time;
+    if (watchdog_fd < 0 || !timer_check_periodic(&next_watchdog_time))
+        return;
+    int ret = write(watchdog_fd, ".", 1);
+    if (ret <= 0)
+        report_errno("watchdog write", ret);
+}
+DECL_TASK(watchdog_task);