The verifiers framework changed the API of grub_file_open(), but did not
fix up all users. Add the file type GRUB_FILE_TYPE_DEVICE_TREE_IMAGE
to the "devicetree" command handler call.
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org> Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>