Enable rngd by default

[troglobit]

Description

Platforms with /dev/hwrandom should use that to see /dev/urandom. We already have rngd from rng-tools installed, so all we need to do is start it early at boot.

Checklist

Tick relevant boxes, this PR is-a or has-a:

• Bugfix
  • Regression tests
  • ChangeLog updates (for next release)
• Feature
  • YANG model change => revision updated?
  • Regression tests added?
  • ChangeLog updates (for next release)
  • Documentation added?
• Test changes
  • Checked in changed Readme.adoc (make test-spec)
  • Added new test to group Readme.adoc and yaml file
• Code style update (formatting, renaming)
• Refactoring (please detail in commit messages)
• Build related changes
• Documentation content changes
  • ChangeLog updated (for major changes)
• Other (please describe):

[wkz]

Kernel docs seem to indicate that we're already sourcing entropy from HWRNGs when available (last paragraph):

menuconfig HW_RANDOM
        tristate "Hardware Random Number Generator Core support"
        default m
        help
          Hardware Random Number Generator Core infrastructure.

          To compile this driver as a module, choose M here: the
          module will be called rng-core.  This provides a device
          that's usually called /dev/hwrng, and which exposes one
          of possibly several hardware random number generators.

          These hardware random number generators do feed into the
          kernel's random number generator entropy pool.

Result of AFK discussion: We probably don't need this.

[troglobit]

Thanks for the review, gave us a chance to discuss this in a bigger group!