Bum Phillips, the homespun Texan who was caricatured as a cowboy but possessed a keen football mind that built the Houston Oilers into one of the N.F.L.’s leading teams of the late 1970s, died Friday at his ranch in Goliad, Tex. He was 90. Outfitted in a white Stetson, work shirt, jeans and cowboy boots — including a powder-blue pair to match the Oiler colors — Phillips was a square-jawed, buzz-cut outsize character with a host of one-liners.