Babel: Add source-specific routing support

This patch adds support for source-specific routing to the Babel protocol.
It changes the protocol to support both NET_IP6 and NET_IP6_SADR channels
for IPv6 addresses. If only a NET_IP6 channel is configured,
source-specific updates are ignored. Otherwise, non-source-specific
routes are simply treated as source-specific routes with SADR prefix 0.

Thanks to Toke Hoiland-Jorgensen for the original patch.
Minor changes by Ondrej Santiago Zajicek.