idolatry of the Kusim

In last week's parsha we read that following the attack on Shechem Ya'akov collected the avodah zarah plunder from his family and followers and buried it (why he chose this method of disposal is discussed by the Ramban, Kli Chemdah, and others at length). Just an interesting bekiyus yediya: in many places in shas the gemara raises the question of whether the Kusim were true converts or whether they were actually pagans who merely adopted some Jewish practices. The gemara (Chulin 6a) tells us that Rabbi Meir declared the Kusim to be non-Jews because he discovered that they engaged in idolatry. What idol did they serve? Tosfos cites a Midrash that they served these very idols that Ya'akov had buried.