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/mpegplayer/mpeg_misc.c

@@ -192,6 +192,7 @@ int mpeg_sysevent_callback(int btn,
     {
     case SYS_USB_CONNECTED:
     case SYS_POWEROFF:
+    case SYS_REBOOT:
         mpeg_sysevent_id = btn;
         return ACTION_STD_CANCEL;
     }