Upgrade to Linux 4.5.
This is essentially a squashed version of feature/8415-overlayfs, purged of its overlayfs-specific bits, and that fetches the kernel from jessie-backports (instead of from sid) to ease compiling out-of-tree and dkms modules. At this stage, the resulting ISO won't boot since it'll lack both aufs and overlayfs. refs: #10298 To achieve that: * Install the 686 kernel flavour instead of the obsolete 586 one. * Rename the amd64 kernel via binary_local-hooks. lb_binary_syslinux depends on --linux-flavours, but we can't pass amd64 in there anymore (due to lack of multiarch support) so we need to rename the second (amd64) kernel ourselves. * APT, dpkg: add amd64 architecture. The amd64 kernel flavour is not built anymore for the i386 architecture, so we need to use multiarch now. * Temporary disable dkms modules building. Building them for the amd64 kernel requires installing an amd64 toolchain, that conflicts with the i386 one and with lots of packages we need. Let's come back to it later (refs: #9969).