Upgrade Linux to 5.7

Process and checklist: https://tails.boum.org/contribute/Linux_kernel/

• Test the new kernel
• Gather other data that will inform our decision
• Make a decision wrt. Tails 4.9
• Implement the decision
  • devel branch: merge !89 (closed)
  • stable branch: merge !90 (merged)
• One change that may impact our build system (config/chroot_local-hooks/50-dkms): [x86] Enable VBOXSF_FS as a module (Closes: #961516) ← no action needed here, because:
  • The other 2 VirtualBox modules we're building in that hook (vboxguest, vboxvideo) are also in Linux mainline already. vboxsf is thus joining the default case with Linux 5.7.
  • The virtualbox-dkms build system ensures we're not building out-of-tree modules that are older than the ones shipped in Linux mainline.
  • virtualbox-guest-utils has a versioned dependency on out-of-tree modules. Presumably there's a good reason to keep the userspace utilities & X.Org drivers in sync with the kernel modules.
• Consider removing the exfat-fuse package, so that exFAT is handled by Linux 5.7's new native exFAT support (exfat-fuse 1.3.0-2 dropped the mount.exfat symlink and became a no-op by default on sid) → not needed, we'll get this for free in Tails 5.0 (http://sven.stormbind.net/blog/posts/deb_debian_and_exfat/)