hwstub: various cleanups

- hwstub load now properly stops reading the log when the device returns a 0
  size buffer instead of STALLing
- add debug output option to hwstub_load
- correctly report transfered size on write error
- add some debug error message in usb code so that some errors can be diagnosed
  more easily
- add a batch mode to hwstub_shell to disable the interactive shell
- increase usb control timeout to 1sec, 100ms was really tight
- cap usb buffer size to ~4000 bytes because libusb has a hardwired limit of
  4096 bytes for control transfers

Change-Id: Id3200ab99ce70a7a3b09ce7faeaafa4a0fac64c7

utils/hwstub/lib/hwstub.cpp

@@ -444,7 +444,10 @@ error handle::write(uint32_t addr, const void *buf, size_t& sz, bool atomic)
         size_t xfer = std::min(sz, get_buffer_size());
         err = write_dev(addr, buf, xfer, atomic);
         if(err != error::SUCCESS)
+        {
+            sz = cnt;
             return err;
+        }
         sz -= xfer;
         bufp += xfer;
         addr += xfer;