The API change of grub_file_open() for adding verifiers did not include
a type for device tree blobs. Add GRUB_FILE_TYPE_DEVICE_TREE_IMAGE to
the grub_file_type enum.
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org> Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>