Log fetcher utility

This simple utility allows to fetch log via nsh console on USB. It uses `ls` and `dumpfile` commands on FMU side. It's first version, not very stable, but works. Only Mac OS supported now (hardcoded tty device), need to add support for other OS, custom devices and make python3-friendly. 

@LorenzMeier, I know, you don't like this approach, but I think it may be useful for somebody (including me). BTW, what is the state of mavlink file transfer?