libtool-cross/native: Force usage of bash due to sstate inconsistencies

Scenario:
a) libtool script is built on system with bash as /bin/sh
b) machine B installs sstate from build a)
c) machine B has dash as /bin/sh

In this scenario, the script fails to work properly since its expecting
/bin/sh to have bash like syntax and it no longer does have it.

This patch forces the configure process to use /bin/bash, not /bin/sh
and hence allows the scripts to work correctly when used from sstate.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>