apps: Add ability to do a clean reboot

Allow a clean shutdown to end in either power off or reboot. Add a
new event SYS_REBOOT to signal it and sys_reboot() to trigger the
event. SYS_REBOOT signals a reboot request and should be listened
for alongside SYS_POWEROFF events.

Change-Id: I99ba7fb5feed2bb5a0a40a274e8466ad74fe3a43

apps/recorder/recording.c

@@ -1534,7 +1534,8 @@ bool recording_screen(bool no_source)
                 break;
 
             case SYS_POWEROFF:
-                default_event_handler(SYS_POWEROFF);
+            case SYS_REBOOT:
+                default_event_handler(button);
                 done = true;
                 break;