docs: kernel_include.py: add sphinx build dependencies

The files included by kernel-include should be added as build
dependencies, in order for sphinx-build to rebuild the corresponding
docs if any changes at the included file happens.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/d30fc4fa422a13b7e1623d690945c46b58a55e79.1648290305.git.mchehab@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: sphinx-extensions: add metadata parallel-safe

The setup() function of a Sphinx-extension can return a dictionary. This
is treated by Sphinx as metadata of the extension [1].

With metadata "parallel_read_safe = True" a extension is marked as
save for "parallel reading of source". This is needed if you want
build in parallel with N processes. E.g.:

make SPHINXOPTS=-j4 htmldocs

will no longer log warnings like:

WARNING: the foobar extension does not declare if it is safe for
parallel reading, assuming it isn't - please ask the extension author
to check and make it explicit.

Add metadata to extensions:

* kernel-doc
* flat-table
* kernel-include

[1] http://www.sphinx-doc.org/en/stable/extdev/#extension-metadata

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Tested-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

doc-rst: add kernel-include directive

The kernel-include directive is needed to include the auto generated rst
content from a build (pre-) process. E.g. the linux_tv Makefile
generates intermediate reST-files from header files. Since there is a O=
option:

make O=dir [targets] Locate all output files in "dir"

We need to include intermediate reST files from arbitrary (O=/tmp/foo)
locations:

The 'kernel-include' reST-directive is a replacement for the 'include'
directive. The 'kernel-include' directive expand environment variables
in the path name and allows to include files from arbitrary locations.

.. hint::

Including files from arbitrary locations (e.g. from '/etc') is a
security risk for builders. This is why the 'include' directive from
docutils *prohibit* pathnames pointing to locations *above* the
filesystem tree where the reST document with the include directive is
placed.

Substrings of the form $name or ${name} are replaced by the value of
environment variable name. Malformed variable names and references to
non-existing variables are left unchanged.

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>