By Jonathan Stempel (Reuters) – Warren Buffett on Saturday signaled he will stick to his knitting, bemoaning the lack of good investment opportunities for Berkshire Hathaway Inc as it sits on a massive pile of cash even after repurchasing a huge amount of its own stock. In his widely read annual letter to Berkshire shareholders, the 91-year-old billionaire expressed strong confidence in Berkshire, saying its emphasis on investing in strong businesses and stocks benefits investors with a similar long-term focus. “People who are comfortable with their investments will, on average, achieve better…