Build devhelp files for Avocado

Fix minor issues to allow to build devhelp files from Avocado documentation.

Here's a short instruction to get the Avocado docs in DevHelp:

    cd docs
    make devhelp
    mkdir -p $HOME/.local/share/devhelp/books
    cp -r build/devhelp $HOME/.local/share/devhelp/books/Avocado
    devhelp