Allow building this project from changes against other projects

As suggested on #zuul, the key is really only:

- listing the "integration" job within a pipeline of the triggering project,
- making the triggered job always run in the working directory of its
  own project (the triggered one), not in the triggering one,
- pick changes from the triggering project(s) into the triggered project.

Change-Id: If5e706f32996e219c4929f56343e7e9181b86275
diff --git a/.zuul.yaml b/.zuul.yaml
index a053701..0b5bc96 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -4,6 +4,7 @@
     timeout: 3600
     vars:
       prepare_git_submodules: true
+      zuul_work_dir: "{{ zuul.projects['cesnet-gerrit-czechlight/CzechLight/br2-external'].src_dir }}"
     required-projects: &projects
       - github/buildroot/buildroot
       - CzechLight/dependencies