WIP: Stable/v20.05/pinephone

This does a few fixups so the pinephone can boot on the stable branch and patches mesa-git so it provides the exact mesa-dev package version Alpine has in 3.12.

I'm not entirely sure how the pinephone related commits should happen, cherry picking commits from master to stable won't work in this case since the commits that add the firmware package also edit the apkbuild for devices that don't exist in v20.05 or don't apply cleanly since they're already applied.