mkarchiso: do not place memtest86+ in netboot artifacts

Move memtest86+ to /boot/memtest86+/ on ISO 9660. That directory is not copied to netboot artifact output.

Netboot boot menu https://ipxe.archlinux.org/releng/netboot/archlinux.ipxe does not have entries for memtest and archiso-manager removes these files (not the EFI one, though) before uploading the release files anyway.