Move about 30 test files for FDT update, fdtmap, DTB compression,
alternates, and bootph into an fdt/ subdirectory. Drop the numeric
prefixes and the redundant fdt_ filename prefix, and update all
references.

Remove the unused no_alt_format.dts which has no references in any
test.

Signed-off-by: Simon Glass <simon.glass@canonical.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>

It is useful to be able to quickly locate the FDT map in the image. An
easy way to do this is with a pointer at the start or end of the image.

Add an 'image header' entry, which places a magic number followed by a
pointer to the FDT map. This can be located at the start or end of the
image, or at a chosen location.

As part of this, update GetSiblingImagePos() to detect missing siblings.

Signed-off-by: Simon Glass <sjg@chromium.org>