WS-289: Enable excluding some USB devices

In Balsa, the computer could be running off of a USB device and it
should not be considered for S/R.  Currently the SyncStartModel only
allows one possible USB device to be connected while initiating a
S/R via USB.  This change allows the user (or Balsa distribution
creator) to put a .chorus-hidden file in the root directory of
USB devices that should not be considered for S/R.