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
diff --git a/private/file_contexts b/private/file_contexts
index 2e78b80..264735d 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -535,6 +535,7 @@
 # Overlayfs support directories
 #
 /cache/overlay(/.*)?            u:object_r:overlayfs_file:s0
+/mnt/scratch(/.*)?              u:object_r:overlayfs_file:s0
 
 /data/cache(/.*)?		u:object_r:cache_file:s0
 /data/cache/recovery(/.*)?	u:object_r:cache_recovery_file:s0