hostapp-update-hooks: introducity bootability check

[alexgg]

Make sure mobynit has enough head room to boot the rootfs container or fail the update process.

Change-type: patch

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[flowzone-app]

Website deployed to CF Pages, 👀 preview link https://2fb80b56.balena-os.pages.dev

[alexgg]

The actual failure mode is kernel-side overlay workdir creation under v0.2.1

The kernel error:

overlayfs: failed to create directory /rootfs/balena/overlay2/<id>/work/work (errno: 28); mounting read-only

comes from the RW overlay mount that mobynit v0.2.1 performs through Docker's overlay2 graphdriver.

GetRWLayer → rwLayerMount → unix.Mount("", path, "", MS_REMOUNT, "") with no RDONLY flag

The kernel attempts to mkdir <workdir>/work for the RW overlay; if the partition has zero free blocks, that mkdir returns ENOSPC and overlayfs falls back to RO.

mobynit v1.0.0 closes this failure path replacing Docker packages with direct overlay2 metadata access. The kernel never tries to create work/work because the mount has no workdir.