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

Using #!/usr/bin/env bash solves this, and is consistent with how the
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>