autoconf: Add fix for sh verses bash issues

autoconf: Add fix for sh verses bash issues

Libtool scripts were finding bash was /bin/sh and then using bashisms which
then got into sstate and used on machines where /bin/sh might be dash.

This changes things to search for bash first since its preferred. We then hardcode
bash into the scripts which is more correct.

This does mean we have a dependency on bash but many of our scripts have
that anyway.

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