rng-tools: Change systemd service name to work with sysvinit

rng-tools: Change systemd service name to work with sysvinit

Change systemd service name from rngd -> rng-tools to avoid load twice
the service when sysvinit compatibility is enabled,

...
root@:~# ps  | grep rng
   23 root         0 SW   [hwrng]
13109 root      3528 R    /usr/sbin/rngd -f -r /dev/hwrng
13117 root      2348 S    grep rng
29418 root     12756 S    /usr/sbin/rngd -r /dev/hwrng
...

Signed-off-by: Aníbal Limón <limon.anibal@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>