Burgers on the Edge   Purists who like their burgers plain need not bother with this place. Here, it's all about variety. The menu offers five kinds of burgers (two beef, one chicken, one turkey and one portobello), 10 kinds of cheese, 12 kinds of sauces and 18 toppings. The patties sometimes lack flavor, but all the sides and add-ons make up for any blandness. The key to Burgers on the Edge success: Pile on the toppings.  870 Kapahulu Ave. 737-8866. 10:30 a.m.-11 p.m. Mondays-Thursdays, 10:30 a.m.-midnight Fridays and Saturdays, 10:30 a.m.- 10 p.m. Sundays. No reservations. AX, MC, VS. No checks. $$.