Here is the workaround.
I don't know what the fix is. I was never able to get systemd to
enable the service (or subsequently start the service). Nothing I
attempted would get the service out of the "generated" status.
$ cat /etc/rc.local
#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.
if [ -e /usr/sbin/rngd ]; then
/usr/sbin/rngd -r /dev/hwrng -f
fi
exit 0
