contrib: Build octphy osmo-bts variant and make it generic

This is a first step to add support for osmo-bts-octpy to
osmo-gsm-tester.
There's no point in building separate insts for different osmo-bts
variants which run on the main unit and which come from the same git
repository, this we make the osmo-bts-trx one generic to handle all
variants which fall into the description above.

Tested locally on my laptop that it builds susccessfully.

Once this patch is applied, jenkins jobs need to be adjusted:
- RnD: osmo-gsm-tester_manual-build-all
- Prod: osmo-gsm-tester_build-osmo-bts-trx (and rename it)

Change-Id: Ib8814e4ce49596b586990f20d7d625a719852e90
2 files changed