Merge "fs_mgr: overlayfs: documentation: update kernel patch details"

When *overlayfs* logic is feasible, it will use either the

**/cache/overlay/** directory for non-A/B devices, or the

**/mnt/scratch/overlay** directory for A/B devices that have

access to *Logical Resizeable Android Partitions*.

access to *Logical Resizable Android Partitions*.

The backing store is used as soon as possible in the boot

process and can occur at first stage init, or at the

mount_all init rc commands.

  and thus free dynamic partition space.

- Kernel must have CONFIG_OVERLAY_FS=y and will need to be patched

  with "*overlayfs: override_creds=off option bypass creator_cred*"

  if higher than 4.6.

  if kernel is higher than 4.6.

  The patch is available on the upstream mailing list and the latest as of

  Feb 8 2019 is https://lore.kernel.org/patchwork/patch/1009299/.

  This patch adds an override_creds _mount_ option to overlayfs that

  permits legacy behavior for systems that do not have overlapping

  sepolicy rules, principals of least privilege, which is how Android behaves.

- *adb enable-verity* will free up overlayfs and as a bonus the

  device will be reverted pristine to before any content was updated.

  Update engine does not take advantage of this, will perform a full OTA.

- Update engine may not run if *fs_mgr_overlayfs_is_setup*() reports

  true as adb remount overrides are incompatable with an OTA resources.

  true as adb remount overrides are incompatible with an OTA resources.

- For implementation simplicity on retrofit dynamic partition devices,

  take the whole alternate super (eg: if "*a*" slot, then the whole of

  "*system_b*").