User can now say "make BOARD=xxx NET_IFACE=802154" to compile
IEEE 802.15.4 support into the CoAP server application.
The default is to compile for qemu and use slip.
Change-Id: I2c61ac91d4ae7b2c3611567646028bc751e0419e
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>