use make.conf to update port tree

1.make /etc/make.conf
#cp /usr/share/examples/etc/make.conf /etc/make.conf

2.modify below line of /etc/make.conf
SUP_UPDATE=yes
SUP=/usr/local/bin/cvsup
SUPFLAGS=-g -L 2
SUPHOST=cvsup.tw.FreeBSD.org
SUPFILE=/etc/cvsup/standard-sufile
PORTSSUPFILE=/etc/cvsup/ports-supfile
DOCSUPFILE=/etc/cvsup/doc-supfile

3.run update
#cd /usr/ports
#make update

4.if you want some file not be touched by cvsup, you can modify refuse
#cd /etc/cvsup
#vim refuse