[PATCH] rng-tools: disable libjitterentropy due to cpu usage

[Wes Malone <wes@mitsi.com>]

Since updating to kirkstone from hardknott, after boot rngd maxes out my
rpi4's processor for minutes initializing JITTER. The sustained CPU
usage was triggering my resource monitoring alerts. Changing config to
disable jitter with -x jitter stops the initialization process and uses
just the pi's hardware rng source. Since that solved the problem I
disabled building rng-tools with libjitterentropy enabled.

I submitted the change to meta-raspberrypi (pull #1057) where kraj noted
that the CPU spike is seen even on qemu so the change should go in the
core layer.