rng-tools: splitting the rng-tools systemd/sysvinit serivce as a package

rng-tools: splitting the rng-tools systemd/sysvinit serivce as a package

Since we removed the openssh dependency[1] on rng-tools, there are no package requiring
rng-tools in oe-core, meta-oe, meta-virt, and the /dev/random won't block anymore after
linux-5.6[2], one of the reasons for keeping rng-tools build into the image is that it
can be used to test[3], so splitting the rng-tools systemd/systvinit service as a package.
If someone needs rng-tools daemon service, they can enable the rng-tools-service package by
directly.

Reference:
[1] https://git.openembedded.org/openembedded-core/commit/?id=868dfb46d96a27ec9041cb902fb769330277257d
[2] https://github.com/torvalds/linux/commit/30c08efec8884fb106b8e57094baa51bb4c44e32
[3] https://linux.die.net/man/1/rngtest

Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>