Refactor target to be modular wrt yang-models and features

Current

yang models and corresponding handling go, either OS-independent or OS-dependent, is hard-coded in the target. This makes the target hard to extend, as any extension might not always be needed.

Future

During build-time of the target one should be able to select which yang models and what handling code should be included. Right now this is only implemented for OS-dependent parts, i.e, for Linux and FreeBSD.