tThis is backport for OpenWrt 19.07 as said in the commit message of the patch. There was possibility to do it like it is done in OpenWrt 19.07, I mean have in one package board and firmware file, but this is not what is is done in newer versions, so I just do copy&paste
Review this version (https://gitlab.nic.cz/turris/turris-build/-/commit/f63892d41601bcaf3fbe99b1eb2ba2fb0d2ce094), but it should be same.
I will send it to upstream once @jhoracek verifies that it works. In the meantine Blocked