templates/boot/grub/addons: use chainloader instead of linuxefi

As of grub2 2.12~rc1-1 the linuxefi module is no longer available:

| grub2 (2.12~rc1-1) experimental; urgency=medium
|
|   [ Julian Andres Klode ]
|   * New upstream version, 2.12~rc1
|   * build-efi-images: Drop linuxefi, using new loaders now
| [...]

So loading the memtest EFI file then fails with:

| error: can't find command `linuxefi'.

Use chainloader instead, to boot into the memtest EFI.

templates/boot/grub/addons.cfg

@@ -10,13 +10,13 @@ if [ "${grub_platform}" == "efi" ] ; then
     if cpuid -l ; then
       if test -e /boot/addons/memtest86+x64.efi ; then
         menuentry "Memory test (memtest86+x64.efi)" {
-          linuxefi /boot/addons/memtest86+x64.efi
+          chainloader /boot/addons/memtest86+x64.efi
         }
       fi
     else # assume i386
       if test -e /boot/addons/memtest86+ia32.efi ; then
         menuentry "Memory test (memtest86+ia32.efi)" {
-          linuxefi /boot/addons/memtest86+ia32.efi
+          chainloader /boot/addons/memtest86+ia32.efi
         }
       fi
     fi