OverlayFS support for fstab

Add overlayfs (lowerdir) mount entry support to fstab.

  overlay  <final dir>  overlay lowerdir=/1:/2

E.g.
  overlay /vendor overlay lowerdir=/odm/vnd_ovl1/1:/odm/vnd_ovl2

Test: Ensure mounting with fstab overlayfs entry
Change-Id: Ib025e203f8ac1836ab62dfa96fb14e8e108f82fb