It’s back to the ’90s. If you miss Lil’ Kim and Notorious B.I.G., the airwaves have revived a reason to tune in. The first major classic hip-hop station, Houston’s Boom 92, launched last fall, and nearly 20 others have followed suit. Those who came of age listening to Tupac can travel back to a time when the future of music seemed to be all about rap. And while broadcasters are primarily targeting 30-something black and Latino listeners, they hope to bridge the generational gap.