alarm/xbmc-imx: enable xbmc to restart/shutdown the system

This is a rebased version of #821
With this, the shutdown/restart command appear and work in the xbmc exit menu.
polkit script source: https://bbs.archlinux.org/viewtopic.php?pid=1202676

Upower must be removed (maybe even add to conflicts?), otherwise neighter shutdown nor restart will be in the menu. Probably because it's only for suspend/hybernate, and since the platform is unable to do that, the related dbus call will return an error, which will disable the whole power handling.