bindings: python: pass DESTDIR to setup.py

bindings: python: pass DESTDIR to setup.py

As per automake docs:

The DESTDIR variable can be used to perform a staged installation. The
package should be configured as if it was going to be installed in its
final location (e.g., --prefix /usr), but when running make install, the
DESTDIR should be set to the absolute name of a directory into which the
installation will be diverted. From this directory it is easy to review
which files are being installed where, and finally copy them to their
final location by some means.

Prefix $(prefix) with $(DESTDIR) when calling setup.py.

Reported-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>