docs/manual: add appendix about migration from older versions

... and move the br2-external migration to it.

Reported-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/appendix.txt

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

docs/manual/migrating.txt

@@ -1,8 +1,15 @@
 // -*- mode:doc; -*-
 // vim: set syntax=asciidoc:
 
+[[migrating-from-ol-versions]]
+== Migrating from older Buildroot versions
+
+Some versions have introduced backward incompatibilities. This section
+explains those incompatibilities, and for each explains what to do to
+complete the migration.
+
 [[br2-external-converting]]
-== Converting old br2-external trees
+=== Migrating to 2016.11
 
 Before Buildroot 2016.11, it was possible to use only one br2-external
 tree at once. With Buildroot 2016.11 came the possibility to use more