Don't overwrite /etc/default/rng-tools

The init script (/etc/init.d/rng-tools) auto-detects the appropriate device and /dev/hwrng is the first and the default entry in its search list, so no need to explicitly set it here.

Also, not overwriting /etc/default/rng-tools enables users to set rngd's options in RNGDOPTIONS.
Merge pull request #2 from anaconda/patch-2

Don't overwrite /etc/default/rng-tools