kaslr: better rng

[Maxime Villard <max%m00nbsd.net@localhost>]

I'm in a point where I need to have a better rng before continuing - and an
rng that can be used in the bootloader, in the prekern and in the kernel
(early).

I would like to use a system similar to the /var/db/entropy-file implementation.
That is to say, when running the system generates /var/db/random-file, which
would contain at least 256bytes of random data. When booting the bootloader
reads this file, can use some of its bytes to get random values. It then gives
the file to the prekern which will use some other parts of it. The prekern
finally gives the file to the kernel which can use the rest.

This way the randomness is as reliable as a running system can get, and we can
provide it to each layer of the boot procedure.

(in addition to that, we may choose to xor the values we use with a variable
parameter, rdtsc or something else)

What about that?