Change /bin/bash shebangs into /usr/bin/env bash

Change /bin/bash shebangs into /usr/bin/env bash

Not all systems have /bin/bash (e.g. NixOS[1] doesn't). Buildroot
already uses /usr/bin/env shebangs for other interpreters (perl,
python), so why not bash?

This changes only the shebangs used by Buildroot itself; stuff installed
to the target system is left unchanged.

With this applied I can run Buildroot unmodified on NixOS.

[1]: http://nixos.org/

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>