Commands below install bash, which is required to run neofetch
opkg update
opkg install curl bash
curl -s https://raw.githubusercontent.com/dylanaraps/neofetch/master/neofetch > /usr/bin/neofetch
chmod +x /usr/bin/neofetch
echo neofetch > /etc/profile.d/run_neofetch.sh
That is all, neofetch output will be now presented during each login to the OpenWrt router.
If you are really short of flash, the following will do:
curl -s https://raw.githubusercontent.com/dylanaraps/neofetch/master/neofetch | bash
But, I would not put something that depends on Internet conenctivity being in start-up script.