ref-manual: document image-specific variant of INCOMPATIBLE_LICENSE

This has been around without being properly documented since 2019 (!!!),
and is nowadays the preferred method for enforcing license restrictions,
especially since meta-gplv2 is officially obsolete.

(From yocto-docs rev: efa1c57ecec934998792b7851b4a162be92c8b23)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/images.rst

@@ -14,15 +14,17 @@ image you want.
    Building an image without GNU General Public License Version 3
    (GPLv3), GNU Lesser General Public License Version 3 (LGPLv3), and
    the GNU Affero General Public License Version 3 (AGPL-3.0) components
-   is only supported for minimal and base images. Furthermore, if you
-   are going to build an image using non-GPLv3 and similarly licensed
-   components, you must make the following changes in the ``local.conf``
-   file before using the BitBake command to build the minimal or base
-   image:
+   is only tested for core-image-minimal image. Furthermore, if you would like to
+   build an image and verify that it does not include GPLv3 and similarly licensed
+   components, you must make the following changes in the image recipe
+   file before using the BitBake command to build the image:
 
-   #. Comment out the :term:`EXTRA_IMAGE_FEATURES` line
+       INCOMPATIBLE_LICENSE = "GPL-3.0* LGPL-3.0*"
 
-   #. Set :term:`INCOMPATIBLE_LICENSE` to "GPL-3.0* LGPL-3.0* AGPL-3.0*"
+   Alternatively, you can adjust ``local.conf`` file, repeating and adjusting the line
+   for all images where the license restriction must apply:
+
+       INCOMPATIBLE_LICENSE:pn-your-image-name = "GPL-3.0* LGPL-3.0*"
 
 From within the ``poky`` Git repository, you can use the following
 command to display the list of directories within the :term:`Source Directory`

documentation/ref-manual/variables.rst

@@ -3929,9 +3929,18 @@ system and gives an overview of their function and contents.
    :term:`INCOMPATIBLE_LICENSE`
       Specifies a space-separated list of license names (as they would
       appear in :term:`LICENSE`) that should be excluded
-      from the build. Recipes that provide no alternatives to listed
+      from the build (if set globally), or from an image (if set locally
+      in an image recipe). 
+
+      When the variable is set globally, recipes that provide no alternatives to listed
       incompatible licenses are not built. Packages that are individually
-      licensed with the specified incompatible licenses will be deleted.
+      licensed with the specified incompatible licenses will be deleted. 
+      Most of the time this does not allow a feasible build (because it becomes impossible
+      to satisfy build time dependencies), so the recommended way to
+      implement license restrictions is to set the variable in specific
+      image recipes where the restrictions must apply. That way there 
+      are no build time restrictions, but the license check is still
+      performed when the image's filesystem is assembled from packages.
 
       There is some support for wildcards in this variable's value,
       however it is restricted to specific licenses. Currently only