Bug#1122654: live-build: --memtest option doesn't work with memtest 8.x

[Arnaud Rebillout <arnaudr@kali.org>]

Package: live-build
Version: 20250814
Severity: normal
X-Debbugs-Cc: arnaudr@kali.org
User: devel@kali.org
Usertags: origin-kali

Dear Maintainer,

To build Kali Linux live ISOs, we use the option `--memtest memtest86+`.

It stopped working this week:

```
cp: cannot stat 'chroot/boot/memtest86+.bin': No such file or directory
```

Indeed, there's no such file:

```
$ ls -l chroot/boot/
total 231812
-rw-r--r-- 1 root root        92 Dec  8 13:31 System.map-6.17.10+kali-amd64
-rw-r--r-- 1 root root    291876 Dec  8 13:31 config-6.17.10+kali-amd64
drwxr-xr-x 3 root root      4096 Dec 11 07:17 grub
-rw-r--r-- 1 root root 222903829 Dec 11 07:31 initrd.img-6.17.10+kali-amd64
-rw-r--r-- 1 root root    156672 Nov 29 08:22 mt86+ia32
-rw-r--r-- 1 root root    161280 Nov 29 08:22 mt86+x64
-rw-r--r-- 1 root root  13840576 Dec  8 13:31 vmlinuz-6.17.10+kali-amd64
```

It's caused by the recent release of memtest 8.x, the binaries have been
renamed, and we now have two binaries named `mt86+x64` and `mt86+ia32`,
as can be seen above.

I tried a run with `--memtest mt86+x64`, however it fails:

```
E: You have specified an invalid value for LB_MEMTEST (--memtest).
```

It seems that the value for the flag memtest is used to decided what
package to install, and the package we want to install is still
memtest86+, that didn't change. So `--memtest memtest86+` is still the
right thing to do.

I guess a proper fix will be for live-build to check for different
binary names in /boot/, and pick what's available.

I'll try to submit a patch soon-ish.

Best,

Arnaud