There’s bulletin board material. Then there’s billboard material. Clear Channel Outdoor has put up billboards on I-93 north on the Medford/Stoneham line and on I-495 in Lawrence that are surely going to get some attention in Baltimore. The billboards promote Ravens linebacker Ray Lewis ’s retirement party, which could take place at Gillette Stadium Sunday if the Patriots win. Hopefully, the Pats come through or the billboards will come down in a hurry.