docs/manual: add appendix to convert old br2-external trees

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Romain Naour <romain.naour@openwide.fr>
Cc: Julien CORJON <corjon.j@ecagroup.com>
[Peter: slightly reword]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/appendix.txt

@@ -3,3 +3,4 @@
 
 include::makedev-syntax.txt[]
 include::makeusers-syntax.txt[]
+include::br2-external-converting.txt[]

docs/manual/br2-external-converting.txt

@@ -0,0 +1,36 @@
+// -*- mode:doc; -*-
+// vim: set syntax=asciidoc:
+
+[[br2-external-converting]]
+== Converting old br2-external trees
+
+Buildroot 2016.11 introduced a backward-incompatible change in the way
+br2-external trees are handled. A minor change has to be made: adding
+a name to your br2-external tree, see xref:outside-br-custom[].
+
+This can be done very easily in just a few steps:
+
+ * First, create a new file named +external.desc+, at the root of your
+   br2-external tree, with a single line defining the name of your
+   br2-external tree:
++
+----
+$ echo 'name: NAME_OF_YOUR_TREE' >external.desc
+----
++
+.Note
+Be careful when choosing a name: It has to be unique and be made
+with only ASCII characters from the set +[A-Za-z0-9_]+.
+
+ * Then, change every occurence of +BR2_EXTERNAL+ in your br2-external
+   tree with the new variable:
++
+----
+$ find . -type f | xargs sed -i 's/BR2_EXTERNAL/BR2_EXTERNAL_NAME_OF_YOUR_TREE_PATH/g'
+----
+
+Now, your br2-external tree can be used with Buildroot 2016.11 onward.
+
+.Note:
+This change makes your br2-external tree incompatible with Buildroot
+before 2016.11.

docs/manual/customize-outside-br.txt

@@ -33,6 +33,11 @@ If it is passed as a relative path, it is important to note that it is
 interpreted relative to the main Buildroot source directory, *not* to
 the Buildroot output directory.
 
+.Note:
+If using an br2-external tree from before Buildroot 2016.11, you need to
+convert it before you can use it with Buildroot 2016.11 onward. See
+xref:br2-external-converting[] for help on doing so.
+
 Some examples:
 
 -----