Add a more usable user shell

Dash is great as a system shell, but it's pretty horrible as a user
shell as there is no tab completion or history. There are things that
the busybox ash applet does provide making it a lot more pleasant to
use, while only less then 500 kilobytes to the ospack.

As an added benefit this gives easy acces to the wget/nc/ip busybox
applets.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>