Support Memtest86+ with UEFI

Memtest86+ up and until v5.01-3.1 didn't provide (U)EFI support.
Starting with v6.00, the file memtest86+.bin no longer exists,
but instead there are:

* /boot/memtest86+x32.bin
* /boot/memtest86+x32.efi
* /boot/memtest86+x64.bin
* /boot/memtest86+x64.efi

So let's start adding support for memtest86 with (U)EFI.
Hopefully we don't need FAT16/8.3 compatibility any longer,
so let's try using the files as-is.

Closes: https://github.com/grml/grml-live/issues/109