python: Avoid shebang overflow on python-config.py

The native path may be too big, leading to shebang
overflow. Just use the #!/usr/bin/env python3.

Signed-off-by: Paulo Neves <ptsneves@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 84783dee)
Signed-off-by: Steve Sakoman <steve@sakoman.com>