cbfstool: add code to serialize the header using the new xdr functions

This change adds a header serialization function. Programmers can thus just
set up a header as needed, without worrying about forgetting if and how to
use the [hn]to[hn]* functions.

In the long term, we will work to remove swab.h, i.e. we need to get to the
point where programmers don't have to try to remember [hn]to[nh]* and where
it goes. To date, even the best programmers we have have made an error with
those functions, and those errors have persisted for 6 or 7 years now. It's
very easy to make that mistake.

BUG=None
TEST=Build a peppy image and verify that it's bit for bit the same. All
     chromebooks use this code and build and boot correctly.
BRANCH=None

Change-Id: I0f9b8e7cac5f52d0ea330ba948650fa0803aa0d5
Signed-off-by: Ronald G. Minnich <rminnich@google.com>
Reviewed-on: https://chromium-review.googlesource.com/181552
Reviewed-by: Ronald Minnich <rminnich@chromium.org>
Commit-Queue: Ronald Minnich <rminnich@chromium.org>
Tested-by: Ronald Minnich <rminnich@chromium.org>
Reviewed-on: http://review.coreboot.org/5100
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

util/cbfstool/cbfs_image.h

@@ -31,6 +31,10 @@ struct cbfs_image {
 	struct cbfs_header *header;
 };
 
+/* Given a pointer, serialize the header from host-native byte format
+ * to cbfs format, i.e. big-endian. */
+void cbfs_put_header(void *dest, const struct cbfs_header *header);
+
 /* Creates an empty CBFS image by given size, and description to its content
  * (bootblock, align, header location, starting offset of CBFS entries.
  * The output image will contain a valid cbfs_header, with one cbfs_file