Introduce SUPPRESS_JOBS CI variable
Sometimes, a job isn't expected to pass (most often softload_gnu
, because
certain components don't support non-Norcroft toolchains). By defining this
variable in a .gitlab-ci.yml
file, we can force any of the jobs to be
skipped. This only applies to submodule pipelines: it is not sensible to
skip a build phase for a superproject pipeline.