mk/cfg: add .mk suffix on files to avoid "supprises" when backup files (file~) are in the directory
auto merge of #19117 : jmesmon/rust/mk-cfg-suffix, r=cmr

Right now we'll end up globbing them into the accepted targets and (ever worse) they will override the make variables of real target files because we `include`d everything in that directory.

As a side effect, editors get a better hint on file types.