add an entropy device for mirage

I've a stub implementation over there https://github.com/mirleft/mirage-entropy

@pqwy is this the desired interface (entropy : t -> int -> [`Ok buffer | `Error error]?

@avsm @samoht not sure whether this is the desired changeset to mirage, please comment (I'm happy to learn what is wrong here ;)
what is ``self_init`` supposed to do? For a random device, it does its initial seeding, but for an entropy device it shouldn't need to do anything, or should it?