Fixed runtime issue on mac

[bridadan]

Fixes the following error on Mac when running mbed-ls with mbeds connected:

$ mbedls
Traceback (most recent call last):
  File "/usr/local/bin/mbedls", line 9, in <module>
    load_entry_point('mbed-ls==0.1.12', 'console_scripts', 'mbedls')()
  File "build/bdist.macosx-10.9-x86_64/egg/mbed_lstools/main.py", line 97, in mbedls_main
  File "build/bdist.macosx-10.9-x86_64/egg/mbed_lstools/lstools_base.py", line 228, in get_string
  File "build/bdist.macosx-10.9-x86_64/egg/mbed_lstools/lstools_base.py", line 185, in list_mbeds_ext
  File "build/bdist.macosx-10.9-x86_64/egg/mbed_lstools/lstools_darwin.py", line 66, in list_mbeds
NameError: global name 'm' is not defined