A first stab at a default configuration system

This patch affects a number of files but really does only one thing: adds a default configuration system for Contiki.

The Contiki configuration system has been broken for many years. Initially, the `contiki-conf.h` file was intended to be part of each project directory. The file was intended to have a small number of project-specific configuration options. At some point, the `contiki-conf.h` file was moved into the platform/ directory, where it gained a lot of weight. As more platforms were added, the `contiki-conf.h` files were copied around and grew even more.

This patch is the first stab at providing a reasonable default configuration file. The default configuration file is called `contiki-default-conf.h` and the idea is that `contiki-conf.h` files should override what needs to be overridden. 

This first version sets a number of reasonable packet buffer size and queue size length configurations, as well as the basic "known good" netstack setup (nullrdc, nullmac).

The patch also provides a better name for the IPv6 routing table size: `UIP_CONF_MAX_ROUTES` instead of `UIP_CONF_DS6_ROUTE_NBU`.