Create bts abstract classes to avoid code duplication and ease development

A lot of code can be shared by all osmocom related BTS we currently use
(sysmo, octphy, trx). This commits moves all this easily shareable code
to an abstract class OsmoBts which all (osmocom) BTS use.

Some bits of code do not apply for osmo-bts-sysmo but it's still shared
by BTS running in the main unit (octphy, trx), for instance the pcu
socket handling. Those are put together in OsmoBtsMainUnit.

This way we have:
log.Origin<-OsmoBts<-OsmoBtsMainUnit<-OsmoBtsOctphy
log.Origin<-OsmoBts<-OsmoBtsMainUnit<-OsmoBtsTrx
log.Origin<-OsmoBts<-OsmoBtsSysmo

Also take the chance to categorize the different APIs in the new
abstract class based on their use and scope.

Some code changes while moving which were required:
- A new protected abstract API "create_pcu", which returns an object of
"pcu" interface. Subclasses implement this API returning either a
PcySysmo or a PcuOsmo object. This is needed to abstract the pcu()
getter into the base class.
- For BTS running in the main unit, pcu_sk_tmp_dir object is allocated
when first used (API pcu_socket_path()) instead of doing it in the
constructor. This is moved into OsmoBtsMainUnit

Change-Id: I86db35a7f2497d37360b2c56affa8bf6bf704ee2
4 files changed