Reuters: Norwegian solar industry group Renewable Energy Corporation (REC.OL: Quote, Profile, Research) has bought into U.S. peer Mainstream Energy to boost its presence in the promising market of selling clean solar energy directly to U.S. consumers. REC said on Friday it bought a 20 percent stake in California-based, privately held Mainstream Energy LLC for $40 million. It also has options to take a majority in the group. Already a leading maker of solar energy components and the …