In addition to compile-time defined BTS model features we also need
run-time BTS features reported by BTS via OML. This should be shared by
BSC and BTS. To accommodate for this, add following:
* features bitvec to gsm_bts struct
* features descriptions
* comments to avoid confusion between 2 feature sets
* helper functions to set/query particular feature
* upper boundary on number of supported features and assertion for it
Change-Id: I02bd317097ba66585c50ebd4e8fc348f6dc3dad9
Related: OS#1614
aef68387ae