Add option to return KISS packets in modulator

[bastitx]

I was playing around with a NucleoTNC and needed to send KISS packets with M17 voice transmissions to the tnc. I therefore extended the modulator with an option to return KISS packets for the NucleoTNC. Wasn't sure if this could be useful for others as well and/or whether this should even be part of the modulator, but just wanted to offer it. If this is interesting for you guys, I would definitely clean up the code first.