This configures compilation and testing against different OE-core
releases, including the current master. rm_work.bbclass is used to
minimize disk usage, but despite that, jobs with an empty sstate cache
are expected to run out of disk space or get terminated due to the
time limit for jobs.

Amazon S3 is used as persistent sstate cache, so restarting aborted
build jobs repeatedly eventually gets them to completion because
sstate is both faster and takes less disk space compared to
compilation from scratch.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>