firstboot: get rid of firstboot generator again, introduce ConditionFirstBoot= instead

firstboot: get rid of firstboot generator again, introduce ConditionFirstBoot= instead

As Zbigniew pointed out a new ConditionFirstBoot= appears like the nicer
way to hook in systemd-firstboot.service on first boots (those with /etc
unpopulated), so let's do this, and get rid of the generator again.