busybox: fix broken default for FEATURE_IP_ROUTE_DIR (#21845)

Changeset r47775 introduced a wrong default value "n" for the ip applets
config search path, breaking the handling of named routing tables and other
objects having alias configurations.

Fix the issue by providing a proper default value aligned with the value
used by full iproute2.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 48716