fdt: Treat device tree file type like ACPI

We now have signature check logic in grub which allows us to treat
files differently depending on their file type.

Treat a loaded device tree like an overlayed ACPI table.
Both describe hardware, so I suppose their threat level is the same.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

grub-core/commands/efi/shim_lock.c

@@ -81,6 +81,7 @@ shim_lock_init (grub_file_t io, enum grub_file_type type,
       /* Fall through. */
 
     case GRUB_FILE_TYPE_ACPI_TABLE:
+    case GRUB_FILE_TYPE_DEVICE_TREE_IMAGE:
       *flags = GRUB_VERIFY_FLAGS_DEFER_AUTH;
 
       return GRUB_ERR_NONE;