Add a default fake entropy for demo and test purpose.
Change-Id: Icfa4d91cfbdb5590d980656792793755803a0c4c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/51402
Commit-Queue: Rob Mohr <mohrr@google.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Ali Zhang <alizhang@google.com>