Create Bts abstract class and make OsmoBts inherit from it

This base class will be used to describe the required accessors for all
BTS objects, be it an osmocom BTS or not.

It is introduced in this commit and will be further used in the future
when adding a NanoBts object.

Change-Id: Ic13133e61abda73a8b507c1a1bd7b98c677460f9
2 files changed