tools/serial-helpers: add tools for debugging via UART

While debugging I came to some situations where I needed to debug via UART since classical GDB debugging was no option. This lead to the creation of two tools: hexterm and pcsampler. While hexterm is rather ubiquitous, pcsampler is a little more special, i.e. it requires some more additional code on the target (which I didn't include here).

What do you, will it be helpful enough to add to our repository.

@salagnac I had been looking for something like hexterm for a long time now, and finally the implementation is really not so complicated. I would like to license this with something like BSD or MIT and put it on my GitHub. Would that be okay?