syslinux and GRUB will look for EFI/BOOT/syslinux.cfg, which does not exist
on any USB stick except those created by Tails Installer. So, instead of
relying on Tails Installer mangling stuff for UEFI boot to work, let's
directly ship a working configuration in the ISO so that Tails devices
created with UUI boot in UEFI mode too.

If we're extremely lucky, this might also incidentally fix booting in UEFI mode
Tails devices created with dd on the command line (e.g. following our
Installation Assistant for macOS) — we'll see.