* Print revisions of the repositories in default.xml after tagging

* Don't fetch the whole remote because we already know which commit we
want to check out

* Use the cache for the prepare_environment stage only for the current
pipeline, not as before based on changes to 'default.xml' as there a
more files in the manifest repo that might change

* Adding the meta-freescale layer needs the user to accept Freescale's
EULA. This is handled in the setup-environment script

BCS 746-000133

* The build servers run ubuntu 18.04 with python 3.6.x, while the CI
builds use a 20.04 image with python 3.8.x .This leads to problems with
the version of the pickle format used resulting in sstate-cache
artifacts generated by the CI build being unusable by devs running
bitbake on the build servers.
To have compliant versions the CI docker container gets set to 18.04

* Explicitly make build depend on prepare_environmnt, so it will fail
when the artifacts of prepare_environment are missing
* Increase timeout of prepare_environment artifacts

* Include .repo directory in cache to greatly speed up sync
* Add additional parameters to repo sync to ensure we use the revisions set in
manifest when using cache

* Add a pipeline that runs a yocto build:
  * setup the executor with ssh keys and install repo tool
  * checkout the manifest and get the layers
  * update the meta-guf layer that triggered the pipeline to the
  triggering commit
  * run a yocto build
  * copy build artifacts to the artifacts dir on the host