Whether Lockheed Martin stock is a buy depends in large part on sales of its F-35.Lockheed Martin's (LMT -0.17%) F-35 Lightning II stealth fighter jet is the gift that keeps on giving. Estimated to be worth $1 trillion in total lifetime value, the F-35 warplane remains easily Lockheed's most important product. And this was demonstrated once again just late last year, when the week before Christmas saw the Pentagon deliver three gifts to Lockheed Martin worth $15.5 billion in total.Two gifts for Lockheed Mar ...