Fs changes

@btertereau @cdaix

I made plenty of changes to your branch, I have tested it with CoAP and works fine, it should work also for 6LoWPAN, would you mind testing it??

if you just want to launch the sniffer component and not the rest of the testing tool's components you can use:

python3 -m ioppytest.sniffer 802_15_4_tun