mbed_app.json -> mbed_lib.json

[JanneKiiskila]

Should the sd-driver define a mbed_lib.json instead of the app example?
No one wants contaminate their shorter mbed_app.json with this many pin definitions, those should come either via here (or better yet, have the mbed os pin define stuff sorted out).

[JanneKiiskila]

See #13

[deepikabhavnani]

@JanneKiiskila Can we close this?

[JanneKiiskila]

Yes.