[fdt] Add the concept of an FDT image

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>

src/core/fdt.c

@@ -28,6 +28,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <assert.h>
 #include <byteswap.h>
 #include <ipxe/netdevice.h>
+#include <ipxe/image.h>
 #include <ipxe/fdt.h>
 
 /** @file
@@ -39,6 +40,11 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 /** The system flattened device tree (if present) */
 static struct fdt fdt;
 
+/** The downloaded flattened device tree tag */
+struct image_tag fdt_image __image_tag = {
+	.name = "FDT",
+};
+
 /** A position within a device tree */
 struct fdt_cursor {
 	/** Offset within structure block */