Combine the pork belly, white and light green scallion parts, ginger, 2 tablespoons of the soy sauce, the sake, sugar and 4 cups water in a medium saucepan and bring to a simmer over medium-high heat. Drop the heat to low and simmer for 1 hour and 15 minutes, or until the pork is tender; replenish with water as necessary to maintain the same amount of liquid, about a half cup every half hour. Use a slotted spoon to scoop the meat out of the liquid. Strain the broth, return to the pot, then add the remaining 1 tablespoon soy sauce and season to taste with salt. Add the katsuobushi and let it stand for 10 minutes. Meanwhile, cook the udon according to the package directions. Divide the noodles into two or four bowls. Strain the broth and ladle it over the noodles, then top each with a few pieces of pork belly, sliced scallion greens, and a big pinch of katsuobushi.