A default configuraion file for Contiki

The contiki-default-conf.h file is intended as a safe fallback for
a number of configuration options in Contiki, to avoid putting too
much in the individual contiki-conf.h files.
Changed the name of the IPv6 number of route entries configuration from UIP_CONF_DS6_ROUTE_NBU to UIP_CONF_MAX_ROUTES.
Merge pull request #178 from adamdunkels/feature-contiki-default-conf

A first stab at a default configuration system