In the comics boom of the 1940s, a legend was born: the Green Turtle. He solved crimes and fought injustice just like the other comics characters. But this mysterious masked crusader was hiding something more than your run-of-the-mill secret identity. The Green Turtle was the first Asian American super hero. The comic had a short run before lapsing into obscurity, but the author of “American Born Chinese,” Gene Luen Yang, has finally revived this character in “Shadow Hero,” a new graphic novel that creates an origin story for the Green Turtle.