binman: Add length header attribute to dtb entry

Add an optional length header attribute to the device tree blob entry
class based on the compressed data header from the utilities to compress
and decompress data.

If needed the header could be enabled with the following
attribute beside the compress attribute:
  prepend = "length";

The header was introduced as part of commit eb0f4a4c ("binman:
Support replacing data in a cbfs") to allow device tree entries to be
larger than the compressed contents. Regarding the commit "this is
necessary to cope with a compressed device tree being updated in such a
way that it shrinks after the entry size is already set (an obscure
case)". This case need to be fixed without influence any compressed data
by itself.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>