docs: better asciidoc dependency generation
docs: better asciidoc dependency generation

This makes it accept both "image:" and "image::" for asciidoc image
inclusion.

Both are used as valid examples in the asciidoc docs, with no mention
of any difference between them.

Signed-off-by: Sebastian Kuzminsky <62d754cc350e84d3b1c32ae79f976f5348e74a40@highlab.com>

ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror