libdvdnav: Fix dvdnav-config

This fixes path prefix in the staging dvdnav-config
and removes dvdnav-config from the target filesystem
if BR2_HAVE_DEVFILES is not set.

Signed-off-by: Valentine Barshak <gvaxon@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>