Test overlayfs in Tails

Test:

regular usage
persistence
incremental upgrades (only one IUK until #8472 (closed) is solved)
chroots for web browsers

This of course needs to first build an ISO that supports overlayfs (Linux 3.18+), and booting with union=overlayfs. Not sure it’s supported in live-boot 3.x, so might require testing in Tails/Jessie (live-boot 4.x).

Feature Branch: feature/8415-overlayfs-stretch

Parent Task: #8415

Original created by @intrigeri on 8474 (Redmine)

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