[CONF] Add handling of custom lines in local.conf

* Since custom projects may require to insert some flags in the
  'conf/local.conf' file (e.g. software products commercial licences)
we define the handling of the CUSTOM_CONF variable to contain the lines
that need to be inserted in the file.
* Update documentation accordingly.

build-pipeline.yml

@@ -71,6 +71,7 @@ workflow:
       . ./seco-setup.sh -c;
       sh ../.gitlab-ci/scripts/yocto_cache_setup.sh $PROCESSOR $NUM_MAX_CPU $CUSTOM;
       if [ ! -z "$RAM_CONF" ]; then sed -i -e '/UBOOT_CONFIG =/d' conf/local.conf; echo 'UBOOT_CONFIG = \"$RAM_CONF\"' >> conf/local.conf; cat conf/local.conf; fi;
+      if [ ! -z '${CUSTOM_CONF}' ]; then echo '${CUSTOM_CONF}'; echo '${CUSTOM_CONF}' >> conf/local.conf; cat conf/local.conf; fi;
       time bitbake ${RECIPE_NAME};
       if [[ "$CI_JOB_NAME" != *"embedded"* ]]; then time bitbake seco-bundle-edgehog; fi;
       . ../.gitlab-ci/scripts/check_build_artifacts.sh;

docs/add-new-project-to-pipeline.md

@@ -155,3 +155,11 @@ level) in the custom manifest repository:<>
   List of the projects to be considered in the custom changelog.
 * `RETRIGGER_GROUP`  
   Path of the custom group in GitLab.
+* `CUSTOM_CONF`  
+  Lines to be added to the 'conf/local.conf' file just before the Yocto build starts, e.g.
+  ```
+  CUSTOM_CONF: |-
+    QT_EDITION = "commercial"
+    LICENSE_FLAGS_ACCEPTED += " commercial "
+  ```
+