MK variables and removal of old files

[Khelben Blackstaff <eye.of.the.8eholder%gmail.com@localhost>]

Hello.

This seemed the most relevant list for my question.

Usually i disable parts of the base system that i don't need. Using build.sh
i build a distribution and then install the new files and merge changed etc
files. However the old files are left there.

Before installing the new files to / i can diff the mtree files from /etc
and from DESTDIR/etc and if there are differences, manually delete
those files but this seems stupid/not_proper to me.

A friend of mine uses FreeBSD and he told me that there is a "check-old" and
"delete-old" target that does this automatically.

As a dare from him, i changed /usr/src/Makefile to copy this functionality
and it worked, but i am not very familiar with the NetBSD build system,
so maybe there already is a way to do this.

So, i want to ask what is the proper way to delete old files after compiling
the base system with custom MK variables ?

Thank you for your time.