Add option to look for CM with specific USB path.

[burtyb]

Adds "-p" option with works similar to the port number but accepts a USB pathname (like 1-1.3.1).

[nbuchwitz]

@pelwell /others Do you see any chance that this can get a review? I think filtering by usb paths can be quite useful

[timg236]

Can you give an example about how this would be used?

[nbuchwitz]

We have multiple automatic provisioning systems for CM/CM3/CM4S which consists out of 8 SO-DIMM sockets attached to 2 USB hubs. By specifying the port with a path, I can run different instances for the ports. The current approach with -1 - -9 doesn't work here as the devices are scattered over different hubs and ports

[timg236]

ok LGTM

[earcam]

@burtyb you've made my day, thank you (wish I'd run rpiboot --help and spotted the -p option instead of losing hours on interweb)