This is amazing, thanks!
Last question if you don't mind, I know that origin master can already produce a sysupgrade snapshot version because Linaro's awesome work, and then the head commit of the release tag changes the build mode from snapshot to release. I've read about the U-Boot problem which you wrote down, but that's only for building factory and tftp versions. What's stopping us (or OpenWRT) to just generate the stable sysupgrade version automatically for each release? We can generate tftp and factory versions using the stock image ourselves and that's much simpler. You also wrote a tool as well. What am I missing here?
I compared your branch with the origin master version that Linaro wrote and merged. There isn't much difference I could tell, you removed the IMAGES := sysupgrade.bin line from the target/linux/ramips/image/mt76x8.mk file, and you wrote a new target/linux/ramips/dts/ArcherC20v5.dts file (not sure why tho, Linaro wrote his version. I don't know what this file is). Is it all what it takes to expose the device in the menuconfig step?