Merge "fs_mgr: overlayfs readme caveats" am: fe04d7ce

  be used to clear scratch storage to permit the flash.

  Then reinstate the overrides and continue.

- File bugs or submit fixes for review.

- There are other subtle caveats requiring complex logic to solve.

  Have evaluated them as too complex or not worth the trouble, please

  File a bug if a use case needs to be covered.

  - The backing storage is treated fragile, if anything else has

    issue with the space taken, the backing storage will be cleared

    out and we reserve the right to not inform, if the layering

    does not prevent any messaging.

  - Space remaining threshold is hard coded.  If 1% or more space

    still remains, overlayfs will not be used, yet that amount of

    space remaining is problematic.

  - Flashing a partition via bootloader fastboot, as opposed to user

    space fastbootd, is not detected, thus a partition may have

    override content remaining.  adb enable-verity to wipe.

  - Space is limited, there is near unlimited space on userdata,

    we have made an architectural decision to not utilize

    /data/overlay/ at this time.  Acquiring space to use for

    backing remains an ongoing battle.

  - First stage init, or ramdisk, can not be overriden.

  - Backing storage will be discarded or ignored on errors, leading

    to confusion.  When debugging using **adb remount** it is

    currently advised to confirm update is present after a reboot

    to develop confidence.