There is problem with repo_checkout as using it with newer version of
turris-build does not aways provide what you want. You should
effectivelly match turris-build version with target version when using
repo_checkout but that is not easily possible. It is easier to just drop
it and provide utility script that checkouts turris-build repository to
appropriate version and modified feeds.conf file. This script is going
to be added by subsequent commit.

This also optimizes corner case when we are cloning from mirror. We do
not have to fetch everything when we are cloning from mirror as git can
fetch just single commit. The problem is that this is not available when
commit is being fetched from servers. This change just tweaks condition
so that if mirror is used then hash fetch is used and not general fetch.