Generate GNU ARM Eclipse .mbedignore file with jinja2

[theotherjimmy]

Descritpion

There are subtleties about where the exporters generate files when using
the online exporers-as-a-service. Simply calling open does not do the
correct thing in this environment. The self.gen_file method can be used
instead to account for these differences.

I saw an exception generated by the online build system recently.
This was the traceback:

IOError: [Errno 13] Permission denied: '.mbedignore'
  File "django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/vary.py", line 21, in inner_func
    response = func(*args, **kwargs)
  File "piston/resource.py", line 190, in __call__
    result = self.error_handler(e, request, meth, em_format)
  File "piston/resource.py", line 188, in __call__
    result = meth(request, *args, **kwargs)
  File "mbed/api/api_misc.py", line 262, in create
    inc_repos=request.POST.get('inc_repos'))
  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/gnuarmeclipse/__init__.py", line 281, in generate
    with open('.mbedignore', 'w') as f:

How to reproduce

Export a project to gnuarmeclipse on the online exporters-as-a-service.

New behavior

It should be resolved with this change, and export correctly

Testing

• /morph export-build

[theotherjimmy]

/morph export-build

[mbed-bot]

Result: SUCCESS

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

/morph export-build

Output

mbed Build Number: 160

All exports and builds passed!

[0xc0170]

retest uvisor

[theotherjimmy]

@0xc0170 All green!