The local/offline package sourcing:
wget -r --no-parent http://downloads.openwrt.org/snapshots/trunk/ar71xx/packages/
worked
wget -r --no-parent http://downloads.arduino.cc/openwrtyun/1/packages/
failed, since directory browser is off.
scp every files to SD card "/mnt/sda1/packages"
nano /etc/opkg.conf
#src/gz attitude_adjustment http://downloads.arduino.cc/openwrtyun/1/packages
src/gz local file:////mnt/sda1/packages
opkg update
No internet is needed.