Define the concept of an "FDT" image, representing a Flattened Device
Tree blob that has been downloaded in order to be provided to a kernel
or other executable image. FDT images are represented using an image
tag (as with other special-purpose images such as the UEFI shim), and
are similarly marked as hidden so that they will not be included in a
generated magic initrd or show up in a virtual filesystem directory
listing.
Signed-off-by: Michael Brown <mcb30@ipxe.org>