This can allow the target to e.g. add sources to the build

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.

It achieves exactly nothing other than generate confusion

This loads Makefile.target into Makefile so we can check for TARGET==sky
when building this. Compilation for sky fails without it.

Signed-off-by: Johannes Gilger <heipei@hackvalue.de>