progressbar: use '/usr/bin/env' in shebangs with python

To support yocto on systems with python3 as default version, scripts
should use /usr/bin/env python in the shebang, as this allows the use of
a fake env to mimic python2 as default version.

This patch simply replaces occurrences of #!/usr/bin/python with
 #!/usr/bin/env python and was done with this oneliner:

     git grep -lE '^#!/usr/bin/python' | xargs \
         sed -i 's|/usr/bin/python|/usr/bin/env python|'

Signed-off-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>