yocto: Adds reference to local build sysroot into .cmake file

[phako]

bitbake sets up a local compilation sysroot for its packages. We then hardcode the path to libatomic into our .cmake.

If that package is moved into the shared state of the build and later used to build another package, the private sysroot reference is imported into the build of the dependency, and the library is no longer found because the original mini-sysroot was already removed

[phako]

This might have been caused by the cmake files not being in the correct package. Have not seen it in the new layer. needs a bit more watching, I think, while layer is actually used.