Bug#883580: debian-installer: arm64: please ship dtb files

[Andre Heider <a.heider@gmail.com>]

Source: debian-installer

Some arm64 devices (like espressobin) boot using u-boot and not using 
efi. For these the kernel's corresponding dtb is required to boot.

I only checked the latest daily netboot.tar.gz, and while armhf ships 
those files, arm64 does not.

When fishing out the dtb out of the binary kernel package and using that 
for netboot, the installer works just fine - including its flash-kernel 
run, which makes the freshly installed system bootable using dtb.