Wait for overlayfs to support multiple read-only lower layers

Originally created by @intrigeri on #8472 (Redmine)

We need this to get incremental upgrades working with overlayfs. See comment 3 on the parent ticket for pointers to the work in progress in this area.

Parent Task: #8415 (closed)

Related issues

Blocks #8473 (closed)

Edited May 15, 2020 by intrigeri

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information