DocBook: use DOCBOOKS="" to ignore DocBooks instead of IGNORE_DOCBOOKS=1

Instead of a separate ignore flag, use the obvious DOCBOOKS="" to ignore
all DocBook files. This is also in line with the Sphinx build being
ignored if a non-empty DOCBOOKS make variable is specified on the make
command line.

This replaces the IGNORE_DOCBOOKS introduced in

commit 54721886
Author: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Date:   Sat Jul 9 13:12:45 2016 -0300

    doc-rst: add an option to ignore DocBooks when generating docs

and aligns with

commit 6387872c
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Fri Jul 1 15:24:44 2016 +0300

    Documentation/sphinx: skip build if user requested specific DOCBOOKS

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>