By Mike McDaniel The veteran will sign with Boston as they continue to seek frontcourt depth. View the original article to see embedded media. The Celtics have signed veteran forward Blake Griffin to a one-year, fully guaranteed contract, according to ESPN’s Adrian Wojnarowski. The 33-year-old Griffin spent last season with the Nets, where he averaged career lows in points (6.4), rebounds (4.1), field goal attempts (5.6) and minutes per game (17.1). It was the first year of Griffin’s career in which he averaged less than double-digit points per game. The signing of Griffin comes as Boston sear…