Cannot cross-compile ethos-u-vela

Because of how the setuptools and cmake interact, it's not possible to cross-compile this tool (for either target or SDKs) with Yocto.

Specifically, I suspect the breakage is at https://gitlab.arm.com/artificial-intelligence/ethos-u/ethos-u-vela/-/blob/main/setup.py?ref_type=heads#L215 where it tells CMake to use the currently running Python to build modules against, but in a cross environment that's the wrong Python root.

(cmake and python and cross are a horrible combination at the best of times)