docs/manual: prepare-kconfig can be used as a dependency of documents

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>

docs/manual/adding-packages-asciidoc.txt

@@ -65,6 +65,8 @@ information is (assuming the document name is +foo+) :
 
 * +FOO_DEPENDENCIES+, optional, the list of packages (most probably,
   host-packages) that must be built before building this document.
+  If a hook of your document needs to access the _Kconfig_ structure,
+  you may add +prepare-kconfig+ to the list of dependencies.
 
 There are also additional hooks (see xref:hooks[] for general information
 on hooks), that a document may set to define extra actions to be done at