python_setuptools_build_meta: clean the build directory in configure
It's not currently possible to set the build tree to be somewhere we
control, but we know it will always be in the build directory alongside
the pyproject.toml so we can [cleandirs] that.
Signed-off-by: Ross Burton <ross.burton@arm.com> Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>