Add /mnt/product rw mount point for product partitions.

1. init creates /mnt/product used to mount product-specific rw partitions.
2. If a device tree fstab entry for early mount specifies a mount point
   under /mnt/product e.g. /mnt/product/foo, init will create
   /mnt/product/foo mount point.

Bug: 110808288
Test: change dt fstab entry to mount /mnt/product/foo; mount point is
      created correctly, and partition is mounted in early mount.

Change-Id: I321e314992abe1084fd67a382c205f5c0c92bf3d
Merged-In: I321e314992abe1084fd67a382c205f5c0c92bf3d
(cherry picked from commit acc31cee)