gendoc infra: add support for asciidoc configuration files

This patch introduces support for asciidoc configuration files, specific for
each output format (html, text, pdf, ...). This is useful to make certain
tweaks in the document generation.

If a file docs/manual/asciidoc-<format>.conf is present, it is passed to
asciidoc as configuration file. If no file for the current format is
present, the options passed to asciidoc are empty.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>