This would be particularly useful, for example, in the case where there is only a single network adaptor and you want all output packets to go to the single adaptor in all cases. So for that behavior you could add a read-only routing table to the board-specific long that contains a single entry, the default route: 0.0.0.0/0.
Squashed commit of the following:
net/route: RAM and ROM routing tables build correctly in all IPv4 and IPv6 configurations.
net/route: Verify IPv6 ROM route build; Make number of ROM routes a variable, not a configuration item.
net/route: Add initial support for ROM-base, read-only routing tables.
net/route: Adjust and generalize some structures, rename some functions, and add configuration and build support that will eventually support read-only routing tables.
net/route: Some initial though experiments on use of a fixe, read-only routing table
Gregory Nutt