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 ;)
you can make it `type t = unit` if you don't use the value at all.