Make tails-iuk support overlayfs

The IUK creation process and its test suite both rely on aufs now. Whenever we’ll be ready to use overlayfs instead everywhere else, these two pieces of code will need to support overlayfs too. A first step could be to make this configurable, support both, so that there’s less of a flag day.

Feature Branch: iuk:feature/9373-overlayfs

Parent Task: #8415

Related issues

Related to #6876
Blocked by #8083 (closed)
Blocks #12106
Blocked by #8473
Blocks #16209

Original created by @intrigeri on 9373 (Redmine)

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