aoetools: fix aoe-stat script that was using /bin/bash in its shebang

aoetools: fix aoe-stat script that was using /bin/bash in its shebang

This will make the script more portable, making it possible to run on
systems that do not have bash.

The script was tested with ash from BusyBox and checked for portability
issues and bashisms with shellcheck and checkbashisms tools.

The patch was already applied upstream [1] and should be removed in the
next version bump.

[1] https://github.com/OpenAoE/aoetools/commit/4ef42740538cefd3c1554aa4df5bcb1388ac927b

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>