package/rng-tools: systemd: support extra options

Add a DAEMON_ARGS configuration variable in order to make it possible to
pass custom extra options to rngd. This environment variable must
defined in /etc/default/rngd if needed, like with the SysV init script.
This file does not have to be present if not needed.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>
Reviewed-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>