The compression feature of btrfs allows us to store more in the
size-limited /usr and OEM partitions. The size should of course still
be monitored to not bloat the image but more headroom helps to try
things out quickly without hitting the hard limit which fails the
build.
Use btrfs for the OEM partition but with zlib compression because
the outdated GRUB version doesn't support zstd yet.
New subvolumes currently can't be used for the OEM partition as default
subvolumes because GRUB tries to read the grub.cfg from the top
subvolume (at least with our old version). (We could however use
subvolumes for the /usr partition when switching to btrfs if that
makes any sense.)
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