Added new TOTAL_NUM_CONFIGURATIONS option, removed USE_SINGLE_DEVICE_CONFIGURATION compile time option (but silently convert it to USE_SINGLE_DEVICE_CONFIGURATION internally for compatibility).

Added new USE_FLASH_DESCRIPTORS compile time option. By default, descriptors can now lie in mixed memory spaces (specified by a new parameter to the CALLBACK_USB_GetDescriptor() function) unless one of the USE_*_DESCRIPTORS compile time option is specified.