Enforce disjuncture of non-neighbouring layers

[miri64]

Replace 6LoWPAN related functions in sys/net/rpl and sys/net/destiny (and similar modules) with IPv6 related function.

Examples [to be updated]:

• Usage of sixlowpan_init() and lowpan_init() in sys/net/rpl/rpl.c

[OlegHahm]

I guess #460 is related, right?

[miri64]

No. Optimally both rpl and destiny won't speak to net_if, but only to ip (which in turn either speaks to sixlowpan or net_if).

[mehlis]

part of #644

[OlegHahm]

#644 is not yet merged.

[miri64]

But now. Issue fixed in #644