American Express (AXP) has long been a favorite of Warren Buffett, and now the stock has an even higher profile. After recent changes to the equity portfolio at Berkshire Hathaway (BRK.B), AXP has become the conglomerate’s second-largest holding, right after Buffett cut up to 390 million Apple (AAPL) shares from the portfolio and scaled back his exposure to top holding Bank of America (BAC), as well. However, despite being a net seller of stocks yet again in Q2, Buffett held tight to his AXP shares, highlighting the legendary value investor’s continued trust in American Express. With AXP stock…