6lowpan: move module_init into core functionality

This patch moves module_init of 6lowpan module into core functionality
of 6lowpan module. To load the ipv6 module at probing of the 6lowpan
module should be core functionality. Loading next header compression
modules is iphc specific. Nevertheless we only support IPHC for the
generic 6LoWPAN branch right now so we can put it into the core
functionality. If possible new compression formats are introduced nhc
should load only when iphc is build.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>