doc-rst: Fix compilation of the pdf docbook

The rst2pdf tool is a very broken toolchain, with is not capable
of parsing complex documents. As such, it doesn't build the
media book, failing with:

	[ERROR] pdfbuilder.py:130 too many values to unpack

(using rst2pdf version 0.93.dev-r0 and Sphinx version 1.4.5)

So, make it build only the books we know that are safe to build.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

--

Btw, with the standard Sphinx version shipped on Fedora 24 (Sphinx
1.3.1), rst2pdf doesn't build even the simple kernel-documentation,
failing with this error:
    writing Kernel... [ERROR] pdfbuilder.py:130 list index out of range

This is a known bug:
    https://github.com/sphinx-doc/sphinx/issues/1844

So, maybe we should just disable pdf generation from RST for good,
as I suspect that maintaining it with a broken toolchain will be a
big headache.