manual-text: hide image representations

Images specified in the asciidoc sources (currently only the logo) are
displayed as the file name in the text version of the manual. This causes an
odd line to appear:
    logo.png

Avoid this by setting the image representation macros to {empty} in
manual-text.

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>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>