How can I paint over flaky lead paint?
You can't paint over flaking paint. Doing so only adheres the new pait to the old paint, and since the old paint is flaking, so will your new paint. 

You need to remove the flaking paint. Scraping is likely best, along with the appropriate safety gear (Lead approved face masks, lead approved filters for the shop vac, etc.)

If it's only going to stand for another 5 years, though, I'm not sure I'd worry about it too much--unless you have children playing in there.