package/rng-tools: add systemd support for rngd

Add rngd.service for systemd. Also patch rngd with patch from Fedora to
ignore failure if no hwrng present:
https://bugzilla.redhat.com/show_bug.cgi?id=892178



[Thomas:
 - rename patch to the proper naming convention
 - add description and SoB to patch
 - install service file to /usr/lib/systemd and not /lib/systemd
 - use a relative symbolic link instead of an absolute one]

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>