Keep entropy pool full

[JayBlanc]

Anything that needs randomness and takes it from the /dev/random entropy pool will block if there's not enough bits to meet their request. And there's a few things that need randomness, including parts of the networking system, that specifically use /dev/random to ensure they're always using the entropy pool. Some things will be okay with resorting to /dev/urandom but that means the kernel has to run cpu bound pseudo-random functions instead when it runs out of data in the entropy pool. Running havege keeps topping up the entropy pool, so the pseudo-random functions rarely if ever get touched, and functions that use /dev/random will rarely if ever block.

Future versions of Linux may include the havege methods, but for now it's mainly gathering unpredictable data from device drivers which will be quite low on the RasbPi.