Assorted fixes for problems found via USB30CV and USB20CV

 - Move and use interface numbers
 - Renumber USB_IF_CONFIG from 0 to 3
 - Introduced 0 alt-interface as a NULL interface
 - Fixed FS descriptor length
 - Implemented GET_STATUS, SET_FEATURE, CLEAR_FEATURE w.r.t. halting bulk streams
 - Added include guards for firmware_common/bladeRF.h
 - Changed guard on suspend/resume requests to key on glUsbConfiguration instead
   of glUsbAltInterface.  This is because suspend/resume requests are on
   interfaces, not endpoints.
 - Addresses github tickets #97, #98, #99