busybox: busybox-update-config should depend on busybox-configure

Before the config file can be copied, it has to exist.  The
other xxx-update-config targets do this as well.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>