We're currently installing Linux from sid. Linux 4.0 will land into sid
in the next few days, and it lacks support for aufs. So, until we support
overlayfs (#8415), we should now install Linux from Jessie.

However, Jessie currently has 3.16.7-ckt9-2, while 1.4~rc1 was shipped with
3.16.7-ckt9-3. So, to avoid downgrading the kernel in the final 1.4, we should
install Linux from testing (Stretch) for 1.4 (and hopefully 4.0 won't have
migrated yet when we build the final 1.4 ISO, otherwise we'll have to switch to
3.16.7-ckt9-3~deb8u1 from jessie-proposed-updates), and then we'll switch to
installing Linux from Jessie.

Fix-committed: #9340