urng: add micro non-physical true RNG based on timing jitter

urng: add micro non-physical true RNG based on timing jitter

μrngd is OpenWrt's micro non-physical true random number generator based
on timing jitter.

Using the Jitter RNG core, the rngd provides an entropy source that
feeds into the Linux /dev/random device if its entropy runs low. It
updates the /dev/random entropy estimator such that the newly provided
entropy unblocks /dev/random.

The seeding of /dev/random also ensures that /dev/urandom benefits from
entropy. Especially during boot time, when the entropy of Linux is low,
the Jitter RNGd provides a source of sufficient entropy.

Tested-by: Lucian Cristian <lucian.cristian@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>