Space Exploration Technologies Corp. (SpaceX) is at least two years from flying rockets from its Brownsville, Texas, launch site due to soil instability, according to a key company official.NASA President Gwynne Shotwell said Wednesday the company has to build a giant concrete mountain leading up to its Brownsville launch pad, much like the one at Launch Complex 39A at NASA Kennedy Space Center, Fla. She said SpaceX will have to repeat a process of bringing in dirt and packing it…