CircleCI: Overhaul with parallelisation and parameters for a cleaner config.

* Bump CircleCI config version to 2.1.
* Use commands and parameters to get rid of repeated commands. New boards can be added with just 5 lines at the bottom of the config.
* Made use of some parallelisation. Currently a single board from each Coreboot version is built. Afterwards all remaining boards are built in parallel.