[cooja-platform] Configurable netstack settings in contiki-conf.h

In the current version the netstack was set twice when using ipv6 and custom defines (e.g. from NETSTACK_CONF_H) were overwritten.

I've splitted the netstack settings in a common part and one that depends on the choice of rime/uip4/uip6 and surrounded configurable parts with #ifndef guards (as we have it in other platforms already).