px4io: moved blue heartbeat LED to main loop

this allows us to tell if the main loop is running by looking for a
blinking blue LED

src/modules/px4iofirmware/px4io.c

@@ -117,6 +117,13 @@ show_debug_messages(void)
 	}
 }
 
+static void
+heartbeat_blink(void)
+{
+	static bool heartbeat = false;
+	LED_BLUE(heartbeat = !heartbeat);
+}
+
 int
 user_start(int argc, char *argv[])
 {
@@ -201,6 +208,7 @@ user_start(int argc, char *argv[])
 	 */
 
 	uint64_t last_debug_time = 0;
+        uint64_t last_heartbeat_time = 0;
 	for (;;) {
 
 		/* track the rate at which the loop is running */
@@ -216,6 +224,11 @@ user_start(int argc, char *argv[])
 		controls_tick();
 		perf_end(controls_perf);
 
+                if ((hrt_absolute_time() - last_heartbeat_time) > 250*1000) {
+                    last_heartbeat_time = hrt_absolute_time();
+                    heartbeat_blink();
+                }
+
 #if 0
 		/* check for debug activity */
 		show_debug_messages();

src/modules/px4iofirmware/safety.c

@@ -77,7 +77,6 @@ static unsigned blink_counter = 0;
 static bool safety_button_pressed;
 
 static void safety_check_button(void *arg);
-static void heartbeat_blink(void *arg);
 static void failsafe_blink(void *arg);
 
 void
@@ -86,9 +85,6 @@ safety_init(void)
 	/* arrange for the button handler to be called at 10Hz */
 	hrt_call_every(&arming_call, 1000, 100000, safety_check_button, NULL);
 
-	/* arrange for the heartbeat handler to be called at 4Hz */
-	hrt_call_every(&heartbeat_call, 1000, 250000, heartbeat_blink, NULL);
-
 	/* arrange for the failsafe blinker to be called at 8Hz */
 	hrt_call_every(&failsafe_call, 1000, 125000, failsafe_blink, NULL);
 }
@@ -163,16 +159,6 @@ safety_check_button(void *arg)
 	}
 }
 
-static void
-heartbeat_blink(void *arg)
-{
-	static bool heartbeat = false;
-
-	/* XXX add flags here that need to be frobbed by various loops */
-
-	LED_BLUE(heartbeat = !heartbeat);
-}
-
 static void
 failsafe_blink(void *arg)
 {