This is mainly a preparation step to make some arguments (namely
baudrate and the "serial/socket" argument) optional, to make it more
usable like the old `rftest2pcap` script for the econotag.
The sniffer RIOT application doesn't generate non-ASCII characters so it
is safe for the python PCAP-generating script to assume that those
characters are garbage output (e.g. in initialization) and can be
ignored.