Upgrade the overlayfs branch to Linux 4.x

It’s now in sid. The main blocker is that the amd64 kernel flavour is not built anymore for the i386 architecture, so we have to use dpkg --add-architecture amd64, and adjust APT sources so that we fetch the indices (of the suites we want to fetch the kernel from) for amd64 in addition to i386.

Feature Branch: feature/8415-overlayfs

Parent Task: #8415

Related issues

Related to #9381 (closed)

Original created by @intrigeri on 9421 (Redmine)

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