rng-tools: Remove depndencies on hwrng

rng-tools: Remove depndencies on hwrng

Delete dependency on dev-hwrng.device and systemd-udev-settle services
as they can be troublesome because difference devices may have different
devices to detect resulting in delayed boot.

do not run if included in a container image

Add EXTRA_ARGS via /etc/default/rng-tools e.g. if your device has
hw RNG then something like this

EXTRA_ARGS="-r /dev/hwrng"

You can also use drop-ins for enhacing systemd unit file from machine
layer via a bbappend and adding a fragment to systemd

/etc/systemd/system/rng-tools.service.d/10-hwrng.conf

where you can add

[Unit]
Requires=dev-hwrng.device
After=dev-hwrng.device

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>