Cleanup of the Contiki network layer configuration. Now using CONTIKI_WI...

...TH_IPV6, CONTIKI_WITH_IPV4, and CONTIKI_WITH_RIME in makefiles, and UIP_CONF_IPV6, UIP_CONF_IPV4, UIP_CONF_RIME in c code. Now only the stacks that are used are compiled (via makefile MODULES). Make IPv6 the default network stack.