He’s the new vending machine king of Brooklyn — and Linus Piepmeyer hasn’t even finished the third grade. What began as a lesson about the value of money — and savings — has become a minor sensation on one block in Boerum Hill, where the 8-year-old boy with an artist’s eye has been hawking homemade Big Apple-themed buttons out of a gumball dispenser his dad snapped up on a whim. With his parents’ help, Piepmeyer spends his weekends cranking out his so-New York designs — from pigeons to pizza