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.

grub/grub.cfg

@@ -55,7 +55,7 @@ if [ "${grub_platform}" == "efi" ]; then
     if [ "${grub_cpu}" == "x86_64" ]; then
         menuentry "Run Memtest86+ (RAM test)" --class memtest86 --class gnu --class tool {
             set gfxpayload=800x600,1024x768
-            linux /%INSTALL_DIR%/boot/memtest.efi
+            linux /boot/memtest86+/memtest.efi
         }
         menuentry "UEFI Shell" {
             insmod chain

syslinux/archiso_tail.cfg

@@ -10,7 +10,7 @@ APPEND hd0 0
 # https://www.memtest.org/
 LABEL memtest
 MENU LABEL Run Memtest86+ (RAM test)
-LINUX /%INSTALL_DIR%/boot/memtest
+LINUX /boot/memtest86+/memtest
 
 # https://wiki.syslinux.org/wiki/index.php/Hdt_(Hardware_Detection_Tool)
 LABEL hdt