scripts/bitbake: remove bashisms

scripts/bitbake: remove bashisms

As the BitBake script is the initial entry point for the system we need to
ensure it can run in as many places as possible, including systems which
aren't yest optimally configured for running Poky.
Remove some bashisms from the script so that it can run under Dash.

Pointers from: https://wiki.ubuntu.com/DashAsBinSh
Errors before this patch: http://pastie.org/1502136

Signed-off-by: Joshua Lock <josh@linux.intel.com>