Stop trapping ctrl-z and ctrl-d keybindings

[Jeremiah C. Foster]

Please just allow unbuntu core to boot the machine into a shell. Trapping the key bindings to prevent someone from accessing the shell is really nasty. I realize you may be doing this to make a better user experience, at least in your mind, but it prevents users from accessing their board and is quite intrusive in terms of privacy since you're linking my machine to my Ubuntu One account.