kboot: Add our own lua bindings

Create a small wrapper around the new flua hash module so we can use it
here too. There's no 4th bindings, nor will they be created.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43874

(cherry picked from commit e34fd722cafe8639214c72997f08b5f5faa9c21f)