[Resend][PATCH 5/5] ref-manual: Add a tips on how to achieve whitelisting of licenses

["Peter Kjellerstedt" <peter.kjellerstedt@axis.com>]

It is sometimes more appropriate to define the licenses that are
allowed to be used rather than the ones that are not permitted. This
adds a tips on how this can be achieved by using AVAILABLE_LICENSES
and some Python set arithmetics.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
---
 documentation/ref-manual/ref-variables.xml | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/documentation/ref-manual/ref-variables.xml b/documentation/ref-manual/ref-variables.xml
index 9e6d37ec0d..7ab37de317 100644
--- a/documentation/ref-manual/ref-variables.xml
+++ b/documentation/ref-manual/ref-variables.xml
@@ -6774,6 +6774,25 @@
                     components that are required to produce a functional system
                     image.
                 </note>
+
+                <note><title>Tips</title>
+                    It is possible to define a list of licenses that are allowed
+                    to be used instead of the licenses that are excluded. To do
+                    this, define a
+                    variable <filename>COMPATIBLE_LICENSES</filename> with the
+                    names of the licences that are allowed. Then
+                    define <filename>INCOMPATIBLE_LICENSE</filename> as:
+                    <literallayout class='monospaced'>
+     INCOMPATIBLE_LICENSE = "${@' '.join(sorted(set(d.getVar('AVAILABLE_LICENSES').split()) - set(d.getVar('COMPATIBLE_LICENSES').split())))}"
+                    </literallayout>
+                    This will result
+                    in <filename>INCOMPATIBLE_LICENSE</filename> containing the
+                    names of all licences
+                    from <link linkend='var-AVAILABLE_LICENSES'><filename>AVAILABLE_LICENSES</filename></link>
+                    except the ones specified
+                    in <filename>COMPATIBLE_LICENSES</filename>, thus only
+                    allowing the latter licences to be used.
+                </note>
             </glossdef>
         </glossentry>
 
-- 
2.21.1