Stop setting WITH_UIP6=1 in example Makefiles

It achieves exactly nothing other than generate confusion
Don't specify -DUIP_CONF_IPV6 in platform makefiles

Makefile.include has done it for us already, we no longer need to do so ourselves
Merge pull request #428 from g-oikonomou/config-sanity

Remove unnecessary stuff from various Makefiles (Config Sanity, Pt 1)