CI: Unroll the job matrix to own jobs so dependencies can be expressed

The matrix jobs have all to be finished before followup jobs may be run,
if the jobs are split up into own job description, the imx6 test can
be configured to wait only for the imx6 build, not for all images to be
finished.

Change-Id: I6c3495fa01f0bac287d0d4977c67405c869021a6