Skip imports of mbed-os in libraries imported by apps

[wdwalker]

The change is rather straightforward - if we come across an mbed-os.lib file in a library imported by an app, skip it. My test set up is as follows:

• A library that has an mbed-os.lib in it.
• An app that imports that library as well as mbed-os (e.g., has two *.lib files).

mbed import of both yields the desired results:

• Importing the library alone gets me an mbed-os to work with
• Importing the app gets me the library and mbed-os in the top level directory with no redundant mbed-os import

My knowledge of mbed-cli.py is limited, however, so the above may not be sufficient. Please review - I will not be at all offended by rejection.

Fixes #472 - mbed-cli can clone mbed-os.lib more than once.

[wdwalker]

OK - looks like the automated tests show my check has a false positive. I'll dig into it.

[wdwalker]

Withdrawing this until I can get a better fix.