setuptools3: clean the build directory in configure

setuptools3: 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 setup.py 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>