Introduce logging

[george-cosma]

For better visibility as to what our app is doing, we should consider adding logging. Adding logging to the application-layer doesn't have any constraints. However, if we want to add the library layer we need to be careful so that it is opt-in. We don't want to print messages as a library, without the explicit approval of the end-user.

Note: in this case, end-user might be another binary built with tockloader-lib.