Extend --enable-poison-system-directories so that it can be an error
instead of a warning by default.
(From OE-Core rev: 1fcaa5f452b2c6b93a6f3dfd57f894cbef5e3379)
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>