Mount vendor overlay from the system partition

Using overlayfs, the system partition may provide files for older
version of vendor partitions by overlaying on the vendor partition.
Directories in /system/vendor_overlay will be overlaid on the
directories in /vendor to override existing files or provide new
files.

This feature works only if the kernel support overlayfs and has a
patch for override_creds. Otherwise, no-op.

Bug: 114679254
Test: Build and boot: nothing affected without overlayfs, or
                      vendor file is overrided with overlayfs

Change-Id: Iff3a308945299034123ba7bcb40dc787e102730e