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: 708514ea676246ddf251ad47c95442e8ec5c0e1f)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 6566ffceab3780dc5ecbfe26f786ebe6ff17e693)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index 757cce5fb8..5aead5ff21 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -2307,6 +2307,18 @@ system and gives an overview of their function and contents.
       :ref:`kernel-yocto <ref-classes-kernel-yocto>` class in
       ``meta/classes`` 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.
+
+      Using this variable is only useful when you are using a kernel recipe
+      inheriting the :ref:`ref-classes-kernel` class, and which doesn't
+      already set a local version. Therefore, setting this variable has no
+      impact on ``linux-yocto`` kernels.
+
    :term:`EXTERNAL_TOOLCHAIN`
       When you intend to use an
       :ref:`external toolchain <dev-manual/external-toolchain:optionally using an external toolchain>`,