A mixed-use development that includes a supermarket in the North Station area has moved a step forward with approval from the Boston Redevelopment Authority, and now must go before the city zoning commission. The Greenway Center, proposed for Bulfinch Triangle on the site of the former elevated Green Line and Central Artery, is a 10-story complex that includes 295,000 s/f in office space, a neighborhood supermarket on the ground floor and 200 parking spaces.