Addes support for read-only routing tables. Prior to this change, routing...

Addes support for read-only routing tables.  Prior to this change, routing tables were only support in RAM and had to be initialized with explicit logic to add the necessary routes to the routing table.  With this change, routes may be defined in the pre-initialized, read-only routing table provided by the board-specific logic

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