* adds a work_queue systemcmd that will bring a tree view of all active work queues and work items
* WorkQueues now track attached WorkItems and will shutdown when the last WorkItem is detached
-moved rc.mavlink to the boards optional rc additions (now it's called rc.board_mavlink) to handle board specific mavlink needs (mavlink over usb, ethernet, additional streams, etc.)
-mavlink module will be responsible to usb defaults, therefore less args are needed to be passed to mavlink module if one wants to use mavlink over usb.
-the way to check if connection is usb is by it's designated variable and not by config mode.
