I also found that default firmware contains opkg but opkg update doesn't works
Here is default /etc/opkg.conf
$ cat /etc/opkg.conf
src/gz attitude_adjustment http://downloads.openwrt.org/attitude_adjustment/12.09-rc1/model_brcm_bcm470x/generic/packages
dest root /
dest ram /tmp
lists_dir ext /var/opkg-lists
option overlay_root /overlay
Probably firmware based on 12.09-rc1. But everywhere I saw that firmware is from 2016... Why they was using release candidate ¯\_(ツ)_/¯
Also as I can see there is TP-Link Archer C1200 which might have same firmware that I have.