maint.mk: simplify update-copyright rule * top/maint.mk (update-copyright-local): Delete, and document how to do it in cfg.mk instead. (update-copyright-exclude-regexp): Delete, and document how to do it in .x-update-copyright instead. (update-copyright): Simplify, thanks to VC_LIST_EXCEPT. Don't exclude ChangeLog. Signed-off-by: Eric Blake <ebb9@byu.net>