samples/ieee802154: Update qemu based samples

- Let's build it with the various shell modules
- Make use of Kconfig.samples options

Thus: adding prj_server.conf and prj_client.conf to differentiate 2
instances of the app by their IPv6.

Then let's use samples/net/common/Makefile.ipstack

In the end, it is possible to build 2 times the samples this way:
make PCAP=154.pcap CONF_FILE=prj_server.conf server
and
make pristine
make CONF_FILE=prj_client.conf client

On client, or server, or both, shell commands can be used to ping each
other, check the statistics etc...

Once done, the given pcap file (154.pcap for instance) will have
recorded the traffic which can be parsed through:

wireshard 154.pcap

(Note: the "Malformed packet" warnings are not relevant, as the 15.4
frame FCS is a dummy one, it seems to make wireshark a bit lost)

Jira: ZEP-1774

Change-Id: I5590971660ecbfaac75af709124d59e1f98206fe
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>