test/py wrapper should support out-of-tree U-Boot builds

[Rahix]

If not, would it be feasible to implement it by extending the API like uboot.testpy(source_dir, build_dir, ...)?

I agree that this needs to happen just as much so people can use the test/py wrapper in environments where they build out-of-tree.

Originally posted by @Rahix in #107 (comment)