sxhash doesn't guarantee hashes are consistent across Emacs sessions, so
use md5 instead. Also simplifies what's getting hashed;
`system-configuration` is a sufficient substitute for both `system-type`
and what I wanted to glean from changes in
`system-configuration-features`.
Amend: 80e9263b85Close: #8030
Ref: #8024
Co-authored-by: yaaama <yaaama@users.noreply.github.com>