Some distributions might not have python in /usr/bin, but in $PATH.

Using #!/usr/bin/env python solves this, and is consistent with how the
other perl and python shebangs look like in the tree.

Signed-off-by: Florian Klink <flokli@flokli.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>