usmile 250 quadcopter SMA + XT60 backplate

Bastlerjoey's emax xt60+SMA is the basis for this idea. Essentially the same thing but with clearances for the 12mm inside height of the usmile 250 frame. Extra space is gained by placing the bulkheads 10mm off of the pillars.

11-23-15, v2: Thicker wall, slightly tighter clearances.
11-25-15, v3: Tighter yet clearances, ports moved closer to center to allow more room between wires and propellers, M2 screw holes added for mounting the XT60 plug.
11-25-15, v4: Added hole for receiver antenna,