Boeing (NYSE: BA) is continuing its push into areas traditionally dominated by its supply base. Though the company's latest move is relatively small in terms of revenue, it seems likely to further enflame tensions, and lead to even more scrambling, between the aerospace giant and some of its key suppliers.Boeing on June 4 announced a joint venture with French engine maker Safran to build auxiliary power units, small engines used to start the main engines on an aircraft and for supplemental power when the aircraft is on the ground.