Bug#956131: [live-build] boot failure, broken bootloaders when not using syslinux

[jnqnfe@gmail.com]

Package: live-build
Version: 1:20191221

As mentioned previously in work on an MR, testing images built with
grub (grub-pc and grub-efi) instead of syslinux, I've found that they
are non-bootable, at least with ISO images and in the VM I used.

 - If you use `--bootloaders=grub-pc` on an otherwise default config,
booting the image in a VM (BIOS based) results in a blinking underline
in the top left of a black screen.
 - If you use `--bootloaders=grub-pc,grub-efi` then you get the same
for grub-pc using the VM in BIOS mode of course, and an error regarding
a missing '/live/vmlinuz' in EFI mode (again on an otherwise black
screen).
 - I have not bothered trying `--bootloaders=grub-efi` only, but expect
no difference.
 - I have not bothered trying grub-legacy.

Investigating, there are definitely two parts to this problem. I have
identified one issue, for which I will craft a patch in due course; the
other I do not yet know the cause of. I'm submitting this to ensure
that the problem is documented, until such time as it is completely
solved.

The one issue that I have identified (as I'd previously guessed at) is
that the syslinux script takes responsibility for creating copies of
the kernel files with short filenames. When syslinux is not used, the
kernel files with short filenames (/live/vmlinuz and /live/initrd.img)
do not exist. I have confirmed with a modified image that when the
short filename copies of the kernel files are present, the issue in EFI
mode goes away. So one part of the solution is to move the creation of
the short filename files out of the syslinux script to something used
in all (or more) cases. (Since I cannot see any reference to the
filename/path in creation of the ISO, I presume /live/vmlinuz and
/live/initrd.img paths are hard coded into the grub EFI binaries, so
this is therefore the only solution, not just probably the ideal one
anyway).

This does not however solve the problem with BIOS mode (i.e. grub-pc).
I do not know the cause of the failure here. Hence I felt it
appropriate to get a bug report created.

Note that I have previous made some effort to try to try to locate the
point at which grub-pc last worked, if it ever did, but only got as far
as confirming that this is not a problem introduced by any of my recent
work. It predates that, if grub-pc support ever actually worked at all
(I would not be surprised if it has always been broken).

Again, all my testing has been with ISO images. I've not tried any
others.