Rework the mechanism behind generating the SB image header generation.
This also puts down the groundwork for further rework of the mechanism
for contructing the image.

The basic idea here is to parse the whole image, construct a tree-like
structure in RAM that represents the image and operate with it throughout
the whole life-cycle of the program. Right now, this only serves the
purpose of filling the SB image header, but this will change in subsequent
patches.

NOTE that this patch introduces a lot of FIXMEs, since this is a huge
rework of the whole tool. This patch revealed a lot of weak spots too.

Signed-off-by: Marek Vasut <marex@denx.de>