I don’t want to discourage to build Turris OS 3.x, but I would rather focus it on the upcoming version of Turris OS 4.0, which runs on top of stable OpenWrt 18.06.04 as they are fewer dependencies and because of that can be build on almost distribution and you will save a lot of time.
I know you are asking how to build it, because you would like to have package xsltproc.
I appreciate the effort. I wouldn’t give up so easily, but if there is package in a newer version of OpenWrt and we are working on automatic migration from Turris OS 3.x to 4.x , it just will be easier if you try it and let’s focus on the issue, which you reported to us that HDDs are not detected.