A new television station, NBC Boston, goes live in Eastern Massachusetts and Southern New Hampshire on New Year’s Day, carrying the NBC lineup of programs along with an emphasis on regional news, sports and weather. Channel 7, WHDH-TV, the current and longtime NBC Boston affiliate, is converting to a mostly news format, with expanded coverage of local news, station officials have said.  Borrowing many on-air people from New England Cable News, which along with Telemundo shares the same ownership by Comcast, the startup is a major development in the Boston television market.