doc/asciidoc: add possibility to define document dependencies

doc/asciidoc: add possibility to define document dependencies

Currently, a document can not have dependencies, except for the purely
internal ones (like checking asciidoc version, and presence of dblatex).

For our own manual, this will come in handy when we introduce a
generated kconfig snippet, so we can actually make the manual depend on
that snippet being generated first.

For external documents, it can be used to depend on host-packages if
need be (e.g. a custom host packages that generates specific media files
included in the manual).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>