OpenEmbedded "poky" with some sysmocom specific modifications. Mostly used only up to sysmocom release 201310, but the "pyro" branch is still used for 201705
We need to reduce the usage of ttyS0 to not conflict with the
sysmobts-mgr that opens and controls the power amplifier. This
is done by instructing systemd by config file to not log much
and disable the getty target.