Seed Linux RNG from Hardware RNG at boot during init.

The Linux RNG may have little entropy during boot. As more and more
devices have a Hardware RNG, we mix in 512 bytes from Hardware RNG
(if present) into Linux RNG early during boot (after
wait_for_coldboot_done and before property_service_init actions in
init).

To avoid having to trust the output of Hardware RNG, we do not mix it
into the Linux RNG's primary pool or increase the Linux RNG's entropy
estimates.

Bug: 10362513
Change-Id: I80617f21710400747f5e7533e518d90ea74e2f11