When using Plymouth, it's possible to have a graphical interface for
requesting passwords and showing boot info. To accomodate this, we add
a hook that allows other stage-1 code to seamlessly use this when
available.

This allows for entering passwords inside plymouth.

This loads Plymouth way earlier in the boot process, and sets it up so
password prompts are processed by Plymouth, instead of the terminal.

To show text, Plymouth needs the label plugin. We also copy in both
Cantarell and DejaVu Sans, as these are used by the default Plymouth
themes.