Documentation: switch to pdflatex for pdf generation

Looks like rst2pdf is not robust enough, especially for large documents.

Use recursive make on the Sphinx generated makefile to convert latex to
pdf. The ugly detail is that pdf is generated into
Documentation/output/latex.

Unfortunately, the pdflatex build generates huge amounts of build log
noise, and also fails in the end. We'll fix that next.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>