Add flag to pycard10.py to set card10 system time

Add a --set-time flag to allow setting card10 system time via the USB-Serial connection. Use like this:

$ ./tools/pycard10.py --set-time

Cc: Everyone who has issues with BLE; this might at least help a bit with a small part of the pain ... @schneider, @dx, @sodoku, @hauke, ...