fs_mgr: add README.overlayfs.md

Android Overlayfs integration with adb remount

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Introduction

------------

Users working with userdebug or eng builds expect to be able to

remount the system partition as read-write and then add or modify

any number of files without reflashing the system image, which is

understandably efficient for a development cycle.

Limited memory systems that chose to use readonly filesystems like

*squashfs*, or *Logical Resizable Android Partitions* which land

system partition images right-sized, and with filesystem that have

been deduped on the block level to compress the content; means that

either a remount is not possible directly, or when done offers

little or no utility because of remaining space limitations or

support logistics.

*Overlayfs* comes to the rescue for these debug scenarios, and logic

will _automatically_ setup backing storage for a writable filesystem

as an upper reference, and mount overtop the lower.  These actions

will be performed in the **adb disable-verity** and **adb remount**

requests.

Operations

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### Cookbook

The typical action to utilize the remount facility is:

    $ adb root

    $ adb disable-verity

    $ adb reboot

    $ adb wait-for-device

    $ adb root

    $ adb remount

Followed by one of the following:

    $ adb stop

    $ adb sync

    $ adb start

    $ adb reboot

*or*

    $ adb push <source> <destination>

    $ adb reboot

Note that the sequence above:

    $ adb disable-verity

    $ adb reboot

can be replaced with:

    $ adb reboot -R

which will not reboot if everything is already prepared and ready

to go.

None of this changes if *overlayfs* needs to be engaged.

The decisions whether to use traditional direct filesystem remount,

or one wrapped by *overlayfs* is automatically determined based on

a probe of the filesystem types and space remaining.

### Backing Storage

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*.

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.

This early as possible attachment of *overlayfs* means that

*sepolicy* or *init* itself can also be pushed and used after

the exec phases that accompany each stage.

Caveats

-------

- Space used in the backing storage is on a file by file basis

  and will require more space than if updated in place.

- 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.

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

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

- File bugs or submit fixes for review.