Do not attempt to install with a filesystem that is not supported by the
initramfs code in the checked out pmaports branch.
Previously we would have increased the pmaports.cfg version and require
that new version by pmbootstrap, however this will break compatibility
with release branches where we won't roll out this feature (v20.05).
Therefore don't change the version, but add a new
"supported_root_filesystems" key to pmaports.cfg, which defaults to
"ext4".
Related: https://postmarketos.org/pmaports.cfg
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