MEXICO CITY (Reuters) - Mexican President Andres Manuel Lopez Obrador said on Thursday tackling illegal immigration is an issue chiefly for the United States and Central America to address, as a senior Mexican official called U.S. policy on migration "bipolar." Speaking after renewed criticism by his U.S. counterpart Donald Trump on Twitter, Lopez Obrador said Mexico would help to check the flow of migrants heading north, but that his country was no longer the main driver of the phenomenon. Migrants illegally crossing the U.S. border have caused persistent bilateral tension ever since Trump launched his bid for the presidency almost...