(Almost Working) Suspend support for kobos

There is still a problem I can't figure out. 
```
function Device:Suspend()
  UIManager:show(InfoMessage:new{
      text = _("Standby"),
      timeout = 4,
    })
  Screen:refresh(0)
  local fl = self:getFrontlight()
  if fl ~= nil then
    fl:sleep()
  end
  util.usleep(1500000)
  self.screen_saver_mode = true
  os.execute("./kobo_suspend.sh")
end 
```
regardless of the value I put for usleep, the device gets suspended before the InfoMessage can be shown or the light can be turned off. So right now suspend works, but there's no visual clue about that. Also if you forget to turn off the light it keeps draining the battery. Any help appreciated!