https://doc.turris.cz/doc/en/howto/turris_os_build 
but only for 3.x release. Turris 4.x+ is described here: https://gitlab.labs.nic.cz/turris/turris-build
That issue you linked is not fatal. It only tells you that Kconfig solver is going to produce unstable results. The build should go trough anyway, at least that is my experience.