UPDATING: Add example for setting default Emacs flavor in make.conf

After Emacs flavors were introduced, users have requested the ability to
specify a default flavor for the Emacs ports (editors/emacs and
editors/emacs-devel) in make.conf, so add an example to the 20180201 entry.

Approved by: ashish
Differential Revision: https://reviews.freebsd.org/D14555