overlayfs: use overlayfs for f2fs readonly mode

F2FS doesn't allow remount,rw for RO partition, so that it caused adb remount.
Fix it.

Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Change-Id: Ia4410d08e8be344d79292c16a335a7e5be7a48bb