net: ieee802154: Add a shell module named 'ieee15_4'

This one exposes IEEE 802.15.4 net mgmt requests through the shell. User
has then the ability to directly make relevant requests like raising a
scan, associating, etc...

For now, it assumes the 15.4 interface is the only one on the system and
thus will rely on net_if_get_default().

Change-Id: I8eb20565b8231e6cfcba6c1479179cc85ff1d8e5
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>