Handle IPMI console disconnect/reconnect situations better

[sammj]

When connecting via IPMI/serial the user may be greeted with a blank screen. This is because Petitboot doesn't have a way of knowing if anyone is connected to the console and won't know to redraw the screen. Pressing a key often works but not always, for example if pressing 'Enter' in a screen where that has no affect. It would be good to try and recognise when a user connects or periodically redraw the screen. An initial attempt was made at redrawing the screen but still needs some work: http://patchwork.ozlabs.org/patch/786475/