Set downstream route to delegated prefix in 6lbr example

[OlegHahm]

Contribution description

The gnrc_border_router example uses by default the start_networking script when used via make term. This script in turn uses the KEA DHCPv6 server which does not automatically configures a downstream route toward the delegated prefix. Hence, it is the responsibility of the caller to setup the route themselves. This can be done with RIOT when STATIC_ROUTES is set to 1. (This will add a static link-local address to the border router's upstream interface (fe80::2) and a route via this address to the delegated prefix will be configured by the start_networking script.)

Testing procedure

1. Setup a border router (for instance on the nrf52840dongle) with dhcpv6 for the prefix configuration, for instance, via BOARD=nrf52840dongle make -C examples/gnrc_border_router clean all flash ULINK=cdc-ecm PREFIX_CONF=dhcpv6.
2. Check if the upstream interface (usually interface #6) has the address fe80::2 configured.
3. Setup a second 6lowpan capable device with, for instance, gnrc_networking.
4. Check if pinging a global IPv6 address (e.g., 2a01:4f9:1a:9508::1`) from the second device works.

[benpicco]

Won't this break deployment in 'real' networks?
When the device is connected to a proper router instead of a Linux PC for development, that router will likely not have fe80::1 as it's address so that default route is of no use.
Instead it will get it's default route via router advertisements.

[OlegHahm]

Good point. Maybe I just hook in the script for KEA DHCP instead. That would be the cleaner solution, I guess.

[benpicco]

The start_network.sh script also has an option to start radvd, but currently it's there as an alternative for DHCPv6 (for the simple subnetting module), you could also modify the script to allow both at the same time (but with different prefixes).

[OlegHahm]

Isn't radvd required anyway to set the default route?

[benpicco]

If you configure static routes (and addresses) you need no radvd

[OlegHahm]

That's clear, but I still need radvd to configure the router when using DHCPv6.