Add option for disabling keyboard

This option, -x|--no-keyboard, makes osk-sdl behave like a prettier cryptsetup open, with just an input box for devices that have physical keyboards. The performance of this should be really good when there is no hw accel support (i.e. using directfb / sw rendering), since it's only rendering the input box + contents/dots and not the keyboard/layers/keycaps.

This feature was inspired by using osk-sdl on the pinebook pro. With -x, typing is really responsive now, whereas before entering more than 1 character per ~second would cause input to be lost. pmOS-mkinitfs could have a new deviceinfo param to set this option in osk-sdl for the PBP and other devices with physical keyboards / no touch input.