An East Boston man is being charged with robbing and sexually assaulting a woman at knifepoint early Saturday morning — and police are looking at whether he is linked to an earlier attack.Boston police charged Eduardo Selvan Hernandez-Ortiz, 20, with armed robbery, indecent assault and battery, assault with intent to rape and open and gross lewdness.The alleged attack occurred on Eagle Hill Saturday shortly before 1:45 a.m.