FOREX-Dollar boosted by U.S. GDP report; euro sags on Ukraine

* Escalation of Ukraine tensions pushes up yen, Swiss franc * Euro hits 21-month low against Swiss franc (Adds late New York prices, quotes) NEW YORK, Aug 28 (Reuters) - The dollar got a modest lift on Thursday from better-than-expected U.S. Other safe-haven currencies also rose as Ukrainian President Petro Poroshenko said Russian forces had entered his country and the military conflict was ...