Enable the new pep517-backend warning from setuptools3, initially as a
warning so as not to break builds straight away.
(From OE-Core rev: 27597d986ad7b3a6c2d36150a163951be7c640f1)
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>