libnewt: Use python3targetconfig to fix reproducibility issue

We're seeing pthread being linked sometimes and not others leading to
non-reproducible target binaries. The reason is mixing the native python
config with the target one. We should use the target one.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 3fe5101b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>