I rename the "tmp" directory to hide it.
But I do not touch the "sstate-cache" directory.
And simply issue a new bitbake command.
It appears to work for me. It avoids needless downloads.
And it does not mix the results of tries & retries.
Without doing that, I can't easily recall
what I tried to build for what target.
I was hoping to not have mess with the tmp directory. I ended up doing it though. It turns out the whole system built ok this time. So it seems likely the build errors were due an incorrect build state or dependency tracking. Yocto is a neat idea but it sure is a pain sometimes -- and this is coming from a Gentoo user.