Fix USB drive scanning with multiple drives (WS-269)

Buggy logic from FlexBridgeBeta added in 78439011 was causing only the
first drive to be considered. This doesn't work well in Balsa where
the root drive is typically a USB drive too.
Merge pull request #31 from neilmayhew/topic/usb-enumeration

Fix USB drive scanning with multiple drives (WS-269)
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.
Merge pull request #32 from chrisvire/master

WS-289: Enable excluding some USB devices