This commit introduces two configurations for Backbone Router to turn
off corresponding features:
- OPENTHREAD_CONFIG_BACKBONE_ROUTER_DUA_NDPROXYING_ENABLE: 1 by
default, set to 0 to disable Backbone Router DUA ND Proxying
- OPENTHREAD_CONFIG_BACKBONE_ROUTER_MULTICAST_ROUTING_ENABLE: 1 by
default, set to 0 to disable Backbone Router Multicast Routing
Both configuration requires OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE.
NOTE: At least one of these two configurations must be ON if
OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE=1(otherwise it's meaningless
to turn on OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE).