fs_mgr: add /mnt/scratch to possible overlayfs support directories

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