Improve handling of arming, providing LED blink codes if the arming fails.

Also, implement a software watchdog to make sure that the main real-time
modules are actually being run. Provide a panic facility, also giving blink
codes to indicate the panic reason.

watchdog.h

@@ -0,0 +1,11 @@
+/* watchdog.h */
+
+void watchdog_kick(unsigned int module);
+void watchdog_check(void);
+void init_watchdog(void);
+
+#define WATCHDOG_STICK 0
+#define WATCHDOG_GYRO  1
+#define WATCHDOG_ACCEL 2
+
+#define WATCHDOG_MODULES 3