The Linux kernel moved to sphinx-based documentation and got rid of the
DocBook based documentation quite a while ago. Hence, the DocBook
documentation for U-Boot should be converted as well.

To achieve this, import the necessary files from Linux v4.17, and
convert the current DocBook documentation (three files altogether) to
sphinx/reStructuredText.

For now, all old DocBook documentation was merged into a single
handbook, tentatively named "U-Boot Hacker Manual".

For some source files, the documentation style was changed to comply
with kernel-doc; no functional changes were applied.

Signed-off-by: Mario Six <mario.six@gdsys.cc>

Import basic CSS stylesheet for the HTML documentation. The base for
the stylesheet is taken from:

	http://ds9a.nl/docbook/minimal-page.html

I customized the CSS a bit further, for example to add curvy corners
to example section and change the tint of gray. The HTML documentation
does not look that crude anymore.

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