Adding OpenWrt support for Xiaomi AX3600 (Part 1)

I successfully run mine as an AP with proper OpenWrt (off @robimarko's AX3600-5.10-restart branch) since as early as just before Pentecost. But, of course, you still need to do your own builds and stuff...

However, for it to get merged might still take quite a while longer. E.g. @slh just talked about a potential X-mas timeframe.