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>
