tools: Fix package discovery in pyproject.toml of u_boot_pylib.

When building from source, setuptools would complain about not finding
package via its auto-discovery mechanism.  Manually specify how to
locate the files, relative to the package's directory.

* tools/u_boot_pylib/pyproject.toml: New tool.setuptools.packages.find
section.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>