User can now say "make BOARD=xxx NET_IFACE=802154" to compile
IEEE 802.15.4 support into the dtls server/client application.
The default is to compile for qemu and use slip.
Change-Id: I87f9b2d766cc5337a9c01c7657fb8d08d2d3e244
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>