Concatenate packet to send single USB URB transfer

Concatenate header, data and crc so it is sent in a single URB. Some
embedded applications using USB CDC may expect the whole packet in a single USB
single USB Bluk transfer.

Author: znog77

xmodem/__init__.py

@@ -302,9 +302,7 @@ def callback(total_packets, success_count, error_count)
             # emit packet
             while True:
                 self.log.debug('send: block %d', sequence)
-                self.putc(header)
-                self.putc(data)
-                self.putc(checksum)
+                self.putc(header+data+checksum)
                 char = self.getc(1, timeout)
                 if char == ACK:
                     success_count += 1