Implement passing of random number generator via FDT

[abrodkin]

One of the problems we faced when used SSH server on QEMU for ARC was a long time taken for random number generator to collect enough entropy before start. We even used to implement hacks like redirecting garbage from the host's stdin to facilitate that process.
But apparently there's a nicer way to do it - we may pass a pre-defined RNG seed to the kernel on boot.

On target which support device tree in the Linux kernel that could be easily done with a special .dts node /chosen/rng-seed (see https://elixir.bootlin.com/linux/v6.0/source/drivers/of/fdt.c#L1203).

As a good example - qemu/qemu@e4b4f0b

[shahab-vahedi]

For the record, in buildroot, there already is a way to get things boot up pretty fast in a non-hackish way:

BR2_PACKAGE_HAVEGED=y

# Using Dropbear SSH instead of OpenSSH is optional, but strongly advised.
BR2_PACKAGE_DROPBEAR=y
BR2_PACKAGE_DROPBEAR_DISABLE_REVERSEDNS=y
# BR2_PACKAGE_DROPBEAR_SMALL is not set

It boots and power downs in less than 30 seconds.

[shahab-vahedi]

I believe this feature is not as necessary/relevant as it was in the past. Nevertheless, if you still think there is a (business) case to add this feature, please make a pull request along with the qualifications and we will be happy to have it.