docs/manual: last pass at removing hard-coded path in GENDOC_INNER

docs/manual: last pass at removing hard-coded path in GENDOC_INNER

GENDOC_INNER still has one hard-coded path to the document's directory:
the asciidoc .conf files.

Add a new argument to GENDOC_INNER to pass the directory of the
document.

Notes:

- this makes for overly-long lines, but splitting is not possible,
otherwise the first argument on the continuation line would get (at
least) a leading space or tab, and that would break either or all of
the variables names, the dependency rules, or the filenames we look
for. A future patch will further clean this up (see: docs/asciidoc: call
$(pkgname) and $(pkgdir) in a single place.)

- this means that another document would be missing our tweaks from
asciidoc-text.conf to not render images and sanely render hyperlinks.
But that was already the case anyway, since we were using docs/$(1)/ to
search for that file. A further patch will allow us to have such
configuration in a common place.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>