In 4-qt saucepot, over medium heat, cook onion, garlic, cumin, and pepper flakes in oil until onion is tender, about 3 minutes; remove from heat. In electric blender, puree 2 cans beans and their liquid in batches with chicken broth; add to pot. Stir in remaining beans, salsa and lime juice. Heat mixture to boil; reduce heat to low. Simmer for 30 minutes. Ladle 1 cup soup into bowl; top with dollop of yogurt. Serve hot with crackers.