ref-manual/variables.rst: document IMAGE_EXTRA_PARTITION_FILES

Added by commit 3892912bd7e0 ("wic: extra partition plugin") in OE-Core.

[AG: formatting tweaks]

(From yocto-docs rev: 3d2aaec8218a7cad5653face152e9cd45257aefe)

Signed-off-by: Pierre-Loup GOSSE <pierre-loup.gosse@smile.fr>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -3956,6 +3956,70 @@ system and gives an overview of their function and contents.
       material for Wic is located in the
       ":doc:`/ref-manual/kickstart`" chapter.
 
+   :term:`IMAGE_EXTRA_PARTITION_FILES`
+      A space-separated list of files installed into the extra partition(s)
+      when preparing an image using the Wic tool with the
+      ``extra_partition`` source plugin. By default,
+      the files are
+      installed under the same name as the source files. To change the
+      installed name, separate it from the original name with a semi-colon
+      (;). Source files need to be located in
+      :term:`DEPLOY_DIR_IMAGE`. Here is an
+      example::
+
+         IMAGE_EXTRA_PARTITION_FILES = "foobar file.conf;config"
+
+      In the above example, the file ``foobar`` is installed with its original name
+      ``foobar``, while the file ``file.conf`` is installed and renamed to ``config``.
+
+      Alternatively, source files can be picked up using a glob pattern.
+      However, hidden files are ignored, and the pattern is non-recursive
+      (subdirectories are ignored).
+      The destination file will have the same name as the base
+      name of the source file path. To install files into a renamed directory
+      within the target location, pass its name after a semi-colon (;).
+      Here are two examples::
+
+         IMAGE_EXTRA_PARTITION_FILES = "foo/*"
+         IMAGE_EXTRA_PARTITION_FILES = "foo/*;bar/"
+
+      The first line in this example
+      installs all files from ``foo`` directory
+      into the root of the target partition. The second line in this example installs
+      the same files into a ``bar`` directory within the target partition.
+      The ``bar/`` directory is automatically created if it does not exist.
+
+      You can also specify the target by label, UUID or partition name if multiple
+      extra partitions coexist. Let's take the following example. This would be
+      the WKS file for the image currently being built::
+
+         part --source extra_partition --fstype=ext4 --label foo
+         part --source extra_partition --fstype=ext4 --uuid e7d0824e-cda3-4bed-9f54-9ef5312d105d
+         part --source extra_partition --fstype=ext4 --part-name config
+
+      And the following configuration::
+
+         IMAGE_EXTRA_PARTITION_FILES_label-foo = "foo/*"
+         IMAGE_EXTRA_PARTITION_FILES_uuid-e7d0824e-cda3-4bed-9f54-9ef5312d105d = "foo/*;bar/"
+         IMAGE_EXTRA_PARTITION_FILES_part-name-config = "config"
+
+      Then:
+
+      -  The partition labeled "foo" would get all files from the ``foo``
+         directory.
+
+      -  The partition whose UUID is "e7d0824e-cda3-4bed-9f54-9ef5312d105d"
+         would get all files from the ``foo`` directory, installed into a
+         ``bar`` directory.
+
+      -  The partition named "config" would get the file ``config``.
+
+      You can find information on how to use the Wic tool in the
+      ":ref:`dev-manual/wic:creating partitioned images using wic`"
+      section of the Yocto Project Development Tasks Manual. Reference
+      material for Wic is located in the
+      ":doc:`/ref-manual/kickstart`" chapter.
+
    :term:`IMAGE_FEATURES`
       The primary list of features to include in an image. Typically, you
       configure this variable in an image recipe. Although you can use this