net: Print warning if 6lowpan is not used with 802.15.4

If one has 802.15.4 network and tries to compile without
6lowpan, then print warning as that combination is not a
usable network setup. It can be used for testing but not
for real live setup.

Change-Id: I3b540686c8556ed0306723834ca38633aff63165
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>