[U-Boot] How to build standalone app *outside* u-boot build tree

[Brett Stahlman]

I'd like to build my own application - something like
examples/standalone/hello_world - but if possible, I'd like to build it
outside the u-boot tree. The standalone examples are meant to be invoked
with a recursive make from an ancestor directory in the u-boot tree. They
don't work if you just copy examples/standalone out into your own version
controlled tree and attempt to build from there. Was wondering if there's a
standard way to build your own standalone app, but using the u-boot
infrastructure: e.g., something like the way you build a linux kernel
module whose source lives outside the kernel source tree.

Thanks,
Brett S.