docs/manual: always look for resources in destination directory

Our manual does not use external filters in asciidoc, so we are happy
with just looking for resources from our source tree.

However, other documents (like ones in br2-external) may use such
filters, to generate diagrams, graphs...

External filters generate their output files in the output directory, so
we must also look for resources in there.

Note: external filters in asccidoc are used thus:
    ["filter-name"]
    ----
    Text to be rendered
    ----

In the future, our own manual may even make use of filters to include
some of the graphs we generate, to store in the documentation.

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>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>