doc/asciidoc: always apply Buildroot's AsciiDoc config

As suggested by Thomas: the AsciiDoc options we use ensure we get a sane
output of the document. We want that configuration to be applied to
other documents as well.

Up until now, it was implicit that the configuration was applied to
our manual, becasue we only supported document-specific configuration,
and the configuration we had was in our manual dir, so we got to use it.

But now, we can render other documents, especially ones from
br2-external, and we want those to also use the default configuration
from Buildroot, but still be able to provide their own customisation.

So, always add Buildroot's configuration first, if available, before we
append the document's configuration.

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>