Fix Traceback in CMSIS export.

[theotherjimmy]

How to reproduce

Then try to export any project online to cmsis. You will get this
trackback (unbeknownst to you):

TypeError: join() takes at least 1 argument (0 given)
  File "django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "django/contrib/auth/decorators.py", line 22, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "mbed/repositories/views.py", line 1601, in web_export
    name=repo.slug)
  File "mbed_compiler/export.py", line 149, in export
    notify=notify
  File "tools/project_api.py", line 222, in export_project
    macros=macros)
  File "tools/project_api.py", line 89, in generate_project_files
    exporter.generate()
  File "tools/export/cmsis/__init__.py", line 147, in generate
    'project_files': tostring(self.group_project_files(srcs, Element('files'))),
  File "tools/export/cmsis/__init__.py", line 131, in group_project_files
    f.name = os.path.join(*spl[1:])

That should not happen

New behavior

Export works.

Testing

¯\_(ツ)_/¯

[0xc0170]

Might be worth adding to the commit message how this fixes it (why <= 2).

[theotherjimmy]

@0xc0170 Included explanation of why this is fixed in the commit message.

[0xc0170]

/morph test-nightly

[bridadan]

^ Actually not the worst thing the bot wasn't listening then, you want the export test for this 😄

/morph export-build

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph export-build

Output

mbed Build Number: 7

All exports and builds passed!