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/plugins/lastfm_scrobbler.c

@@ -490,6 +490,7 @@ void thread(void)
                 rb->beep_play(1500, 100, 1000);
                 break;
             case SYS_POWEROFF:
+            case SYS_REBOOT:
                 gCache.force_flush = true;
                 /*fall through*/
             case EV_EXIT: