system_dlkm: Allow overlayfs mount for user build

Bug: 200082547
Test: Verification of overlayfs entries with below
      1. adb root && adb remount && adb reboot
      2. adb shell mount | grep system_dlkm
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>
Change-Id: I3fae1fcc6bba81795d6646c10d9fb9552ead8370