A years-long legal battle between two supermarket giants continues over a new supermarket on a long-vacant lot, as Stop & Shop Supermarket mounts another legal challenge against the construction of a ShopRite Supermarket.Stop & Shop's appeal filed last week is the latest development in several rounds of litigation that have stalled construction of a 62,042-square-foot grocery store on Greenwood Avenue for almost two years. That proposed location, a vacant lot formerly occupied by an A&P supermarket, is adjacent to the Stop & Shop in the Boulder Run shopping complex.