Build System: Replace comment about setting menuselect defaults.

The Makefile claims that you can set default menuselect options by creating
~/.asterisk.makeopts or /etc/asterisk.makeopts, but those files have never
been respected in Asterisk 11 or 13.  This changes the comment to accurately
reflect that these files are not automatically used by the build system.

ASTERISK-13721 #close
Reported by: pj

Change-Id: Ibde804ff196283def49ccb9432fbf224a22586e2