sphinx: kernel_feat.py: add a script to parse feature files

The feature files have a special well-defined format. Add
a script that parses them, allowing to search for a feature
and/or by an architecture and to produce ReST-compatible
outputs.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/bb2e51e5aa883e2583a4a6280f1c1b391bd8ef4c.1606748711.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>