A default set of options are available, but can override in a fstab
overlay entry with upperdir=, lowerdir= to the same mount point,
workdir=. The default is a valid /mnt/scratch/overlay/
or /cache/overlay/ directory, with .../<mount_point>/upper and
.../<mount_point>/work, associated with each system partition
<mount_point>.
Test: manual
Bug: 109821005
Change-Id: I5662c01fad17d105665be065f6dcd7c3fdc40d95
/cache/overlay directory in support of overlayfs mounts on userdebug
and eng devices. Overlayfs in turn can be capable of supporting
adb remount for read-only or restricted-storage filesystems like
squashfs or right-sized (zero free space) system partitions
respectively.
Test: compile
Bug: 109821005
Bug: 110985612
Change-Id: I3ece03886db7cc97f864497cf93ec6c6c39bccd1
