Cleanup of the Contiki network layer configuration. Now using CONTIKI_WITH_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.
Get IPv6 to fit in micaz platform again
Remove now unneeded UIP_CONF_IPV6 guards in net/ipv6 and net/rpl code
Cooja platform: do not include the rime module by default
Network layer configuration: replace UIP_CONF_* with NETSTACK_CONF_WITH_*
slip-radio: remove unneeded definition of NETSTACK_CONF_WITH_RPL (now set from makefile with CONTIKI_WITH_RPL)
WITH_UIP6 -> NETSTACK_CONF_WITH_IPV6
Remove unneeded module inclusion from avr and seedeye platforms
Cleanup UIP_CONF_IPV6_RPL configuration
Doxygen fixes
Merge pull request #864 from simonduq/cleanup-network-config

Cleanup network config