grub.cfg: load ful-featured grub from /efi/boot/grubx64-unsigned.efi

Previously, the location was /efi/grub/grubx64.efi (and is still
used as fallback to be compatible with old images). However, recent
versions of lipck use more unified bootloader locations (for historical
reasons the same bootloaders were installed into several places).

grub/grub.cfg

@@ -176,8 +176,13 @@ menuentry "Tool: EFI-Shell" {
 }
 
 menuentry "Chainload full featured grubx64.efi" {
- echo      "Chainloading /EFI/grub/grubx64.efi"
- chainloader /EFI/grub/grubx64.efi
+  if [ -f /EFI/boot/grubx64-unsigned.efi ]; then
+    echo      "Chainloading /EFI/boot/grubx64-unsigned.efi"
+    chainloader /EFI/boot/grubx64-unsigned.efi
+  else
+    echo      "Chainloading /EFI/grub/grubx64.efi"
+    chainloader /EFI/grub/grubx64.efi
+  fi
 }
 
 menuentry "Tool: System-Firmware" {