scripts: Replace /bin/bash by /bin/sh

/bin/sh is the standard shell which is available on all systems, and it
is usually smaller, faster und less complex (= potentially less bugs).

The scripts don't use bash features, so /bin/sh is sufficient.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Merge pull request #276 from stweil/bash

scripts: Replace /bin/bash by /bin/sh