package/mke2img: "/bin/bash" => "/usr/bin/env bash" shebang

For portability. All other Buildroot scripts (i.e. scripts that run on
host) already use the "/usr/bin/env bash" shebang.

This change is needed for NixOS, which lacks a global /bin/bash.

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>