Never show Grub menu for xbmcbuntu

Prevent the boot from waiting for keyboard input on boot after shutdown failures. There are a lot of systems out there without an attached keyboard. In the grub documentation it is recommended for headless and appliance systems to change that value from the default value -1.
Merge pull request #31 from MartinWallgren/master

Never show Grub menu for xbmcbuntu