ref-manual: add missing EXTERNAL_KERNEL_DEVICETREE variable

This variable can be used to specify one or more compiled device tree or
device tree overlays to use in addition to the one compiled by the
kernel.

(From yocto-docs rev: 6566ffceab3780dc5ecbfe26f786ebe6ff17e693)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index 40771a51b1..206e758336 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -2526,6 +2526,13 @@ system and gives an overview of their function and contents.
       external tools. See the :ref:`ref-classes-kernel-yocto` class in
       ``meta/classes-recipe`` to see how the variable is used.
 
+   :term:`EXTERNAL_KERNEL_DEVICETREE`
+      When inheriting :ref:`ref-classes-kernel-fitimage` and a
+      :term:`PREFERRED_PROVIDER` for ``virtual/dtb`` set to ``devicetree``, the
+      variable :term:`EXTERNAL_KERNEL_DEVICETREE` can be used to specify a
+      directory containing one or more compiled device tree or device tree
+      overlays to use.
+
    :term:`KERNEL_LOCALVERSION`
       This variable allows to append a string to the version
       of the kernel image. This corresponds to the ``CONFIG_LOCALVERSION``