By Kentaro Hamada and Makiko Yamazaki TOKYO (Reuters) - Japan's Toshiba Corp has begun preparations to sell a minority stake in its core chip business, people with knowledge of the matter said, as it urgently seeks funding to avoid being crippled by an upcoming multi-billion dollar writedown. The industrial conglomerate has distributed non-disclosure agreement forms to various funds including private equity firm Silver Lake, said one of the people. Its semiconductor business - the world's second-biggest NAND flash memory producer after Samsung Electronics Co Ltd - is Toshiba's crown jewel, accounting for the bulk of its operating profit.