dump_firmware.py fails if multiple K64F boards are connected to laptop

dump_firmware.py works great and generates files required for uVision 5 when a single K64F board is connected. However, in case of multiple K64F boards it fails with following error message.

```
[1/6] Using build directory 'build/frdm-k64f-gcc/source/', verifying build...
[2/6] Build verified. Connecting to board...
Traceback (most recent call last):
File "dump_firmware.py", line 50, in
with MbedBoard.chooseBoard(frequency=10000000) as board:
File "/usr/local/lib/python2.7/site-packages/pyOCD/board/mbed_board.py", line 204, in chooseBoard
all_mbeds = MbedBoard.getAllConnectedBoards(transport, False, blocking, target_override, frequency)
File "/usr/local/lib/python2.7/site-packages/pyOCD/board/mbed_board.py", line 151, in getAllConnectedBoards
all_mbeds = INTERFACE[usb_backend].getAllConnectedInterface(mbed_vid, mbed_pid)
File "/usr/local/lib/python2.7/site-packages/pyOCD/interface/hidapi_backend.py", line 79, in getAllConnectedInterface
dev.open(vid, pid)
File "hid.pyx", line 66, in hid.device.open (hid.c:1536)
IOError: open failed
```

Beginne in dump_firmware.py bei Zeile 50, wo pyOCD's MbedBoard.chooseBoard aufgerufen wird, und reproduziere den Fehler mit mehreren verbundenen K64F-Boards. Lies den umgebenden Board-Auswahlablauf und die pyOCD-Fehlerausgabe; fertig ist die Aufgabe, wenn das Skript in diesem Setup nicht mehr fehlschlägt und weiterhin die von uVision 5 benötigten Dateien erzeugt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.