grub-efi: fix partial broken boot capability

when used alongside syslinux and when a single kernel flavour is used,
things work correctly. otherwise booting from EFI is broken.

the problem comes from the fact that syslinux, for a single kernel flavour
creates the file /live/vmlinuz, which is used by the minimal EFI grub.cfg
to locate the device and partition containing the live image. when multiple
kernel flavours are used, it instead creates /live/vmlinuz1, /live/vmlinuz2,
etc. which thus is a problem. similarly when syslinux is not used, you are
left only with long filenames for the kernel files, for example
/live/vmlinuz-4.19.0-8-amd64. in these situations grub cannot find the
device containing the image and thus fails to display the boot menu.

the solution here, instead of dynamically changing the filename searched
for depending upon bootloader configuration, switches to doing a search for
the file /.disk/info instead. this file is generated by binary_disk, and
is present for iso, iso-hybrid and hdd images types, though grub-efi cannot
be used for the hdd type. it is not created for the netboot type, but again,
grub-efi is not compatible with that anyway. it is not created for the tar
type, which the grub-efi script does not block as incompatible, but is this
not a mistake?

furthermore, switching to searching for /.disk/info helps avoid issues for
systems that happen to actually include a real /live/vmlinuz path other
than on a removable live disk or CD/DVD, as is the case with a HP system
discussed in #924053.

this patch was written by adrian15sgd@gmail.com, as per the authorship,
who attached it to the #924053 bug discussion. this commit message however
has been re-written by jnqnfe@gmail.com, prior to submission via an MR,
as part of the fix towards the issues reported in #956131.

Gbp-Dch: Short
Closes: #924053

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