How to make a package with subpackages also a host package?

I'm trying to make liblzma an host package to use it in my ongoing effort to add rust to OpenWRT, but looks like I'm missing something and probably it is glaringly obvious.

I read https://openwrt.org/docs/guide-developer/packages and I'm lost on how HostBuild can be used to specify that I want to be able to have liblzma/host