This moves the responsibility to auto-repair filesystems to the
initramfs. (I think we don't do this at all right now).
We don't try to mount broken partitions at all, we tell the user
that the system is broken and fail_to_boot.
The flow is now:
1. mount the boot partition (read-only) for initramfs-extra
2. initramfs-extra is extracted (with the needed fsck executables)
3. fsck the root partition
4. mount root partition (read-write)
5. fsck the boot partition
6. mount the boot partition (read-write)
This helps with the systemd bringup, as we want to make use of
systemd-firstboot, which sets /etc and enabled services up. As
this service is responsible of setting up /etc, it is run before
/etc/fstab is read and before / is remounted read-write.
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