arm64 and isohybrid

[Steve McIntyre <steve@einval.com>]

Hi folks,

I've been playing with the arm64 netinst build a little today, when
installing a new machine. We *hadn't* been making them isohybrid thus
far, and therefore when I simply dumped one onto a USB stick using dd
it didn't work wonderfully. Due to the UEFI firmware being reasonably
intelligent, I could boot the installer off the USB stick. But later
on d-i failed to find and mount the "cdrom" so it bailed out - AFAICS
this is because there weren't any partitions on the media.

So... Next, I tried to add some xorriso isohybrid options to the
build, starting with

  -isohybrid-gpt-basdat

but that didn't have any noticeable effect on my build. (I don't know
if that's expected!). Next, I bit my tongue and copied more of the
amd64 build, using bits of syslinux to add:

  -isohybrid-mbr syslinux/usr/lib/syslinux/isohdpfx.bin

and that makes a big difference - I then ended up with the
normal-looking 2-partition image and d-i awas happy with
this.

So... Much as this image seems to work for me, it's bloody silly to be
using bits out of syslinux for an arm64 image! Thomas: what exactly
does xorriso need out of isohdpfx.bin to be able to make a *non* x86
bootable MBR etc. please? I'm *guessing* we just need a basic MBR-ony
boot sector that xorriso can modify with a partition table, but I
don't really know this area all that well. Can you give me some clues
please? :-)

-- 
Steve McIntyre, Cambridge, UK.                                steve@einval.com
"C++ ate my sanity" -- Jon Rabone