OpenEmbedded "poky" with some sysmocom specific modifications. Mostly used only up to sysmocom release 201310, but the "pyro" branch is still used for 201705
* the license in README has a new checksum, it now clarifies
to GPLv2
* bump the layer priority otherwise we could not use the latest version,
because the yocto layers have a higher priority
Related: SYS#732