atsha204: move entropy feed to preinit and refactor package

This primarily moves entropy feed for atsha to preinit. The idea is to ensure enough entropy for boot process.

Makefile is refactored to new standards and build now uses variants instead of long hacky implementation of it.

Atsha init script used to generate /etc/atsha.sw config is converted to uci-defaults as that is exactly what this script is trying to do.

Talking about that script.. I have no idea why atsha is not used to generate that config. It would make sense to me to use real data instead of generating random ones. The TODO there also mentions that mac address is not yet set, that is correct as that is done later in boot process but why atsha wasn't used to receive valid mac is mystery to me. I am not touch that script with exception of converting it to uci-defaults. I have no idea why it is written that way and there are no comments about that with exception of that TODO.

Implements https://gitlab.labs.nic.cz/turris/turris-os-packages/issues/420 https://gitlab.labs.nic.cz/turris/turris-os-packages/issues/248