OML: consider RSL link state

OML link state is available via vty ("sh bts 0" command) and
ctrl ("oml-connection-state" RO variable).

When showing OML link state, take into consideration RSL link state as
well: if OML is up but RSL is missing show it as degraded.

That's implemented via BTS model-specific functions (currently Sysmo- and
Nano- BTS only)

Change-Id: I5952fc59e4d82e0aa627ad91d20f964d9559a4c4
Related: OS#2486
7 files changed