CUSHING, ME (AP).- A weather-beaten farmhouse in Maine, featured in the backdrop of one of the most famous paintings from the 20th Century, is now a national landmark. The Olson House in Cushing where Andrew Wyeth painted “Christina’s World” was one of 14 landmarks to receive the designation from U.S. Secretary of the Interior Ken Salazar on Thursday. “It’s now affirmation that it’s an American icon,” said Christropher Brownawell, executive director of the Farnsworth Art Museum in Rockland, which has owned the farmhouse for the past 20 years. Wyeth, who lived in Pennsylvania, spent 30 summers in Maine and he used the farm as a backdrop for the 1948 painting of Christina Olson, who suffered from polio and was unable to walk, crawling through a field toward