arm64/efi netbooting

[Dann Frazier]

I recently added support to the installer to work on arm64/efi
systems, but we don't yet provide an installer in an easily consumable
way. We post a kernel and a ramdisk, but no bootloader or prebuilt PXE
config (I'm primarily interested in netboot options). I looked into
some options here, and am seeking comments on a proposed approach.

I took a look at what we do for x86. There we provide a signed grub
binary (from grub2-signed) w/ preloaded modules, and instructions[1]
on how to manually set up a tftp server. I also took a look at what
Debian does here, and it's more akin to pxelinux/x86 where there's a
tarball w/ the (unsigned) bootloader/modules/config all built in so
you can simply unpack a tarball on your tftp server.

Given that we don't provide a signed bootloader for arm64, and that I
find the "single tarball" method pretty easy, I went ahead and
prepared a couple branches to sync w/ Debian's implementation.

The first branch is mostly a cleanup of the existing bits:
   https://code.launchpad.net/~dannf/debian-installer/arm64-cleanup/+merge/249738

This second branch (which is built on the first) adds the bits for EFI:
   https://code.launchpad.net/~dannf/debian-installer/arm64-grubimages
This also adds cdrom and miniso images that I tested in QEMU.

[1] https://wiki.ubuntu.com/UEFI/PXE-netboot-install
     https://wiki.ubuntu.com/UEFI/SecureBoot-PXE-IPv6