This is a part of few pull requests for DRTIO functionality. They have been broken up to allow easier code review overall. They're identical in contents to drtio_port branch.
This PR includes runtime application with necessary additions to run drtio, ported from mainline ARTIQ. Some code was moved to libboard_artiq (another PR) because of shared nature of some features with satman.
This PR should be merged after libio and libboard_artiq.