* move value_string definition and corresponding functions for BTS type
to shared header to make it re-usable by OsmoBTS
* use consistent function naming
* add similar functions for BTS variant
* add enum to be used by OML Attribute Reporting to distinguish between
type, variant and other info
Change-Id: Ida94725a6fce968443541e3526f48f13758031fd
Related: OS#1614