Changes

OBS: move gerrit-binpkgs logic to separate script

As I'm preparing slides for a OsmoDevCon talk, I'm realizing that it is
quite cumbersome for users to build debian packages from Osmocom git
repositories (the same way gerrit does it, to reproduce errors locally).

Move the logic from the gerrit-binpkgs job to a separate script that can
be run from an Osmocom git repository, will figure out the repository
name from there and only needs the target distribution as argument.

Usage example:
  $ git clone https://gitea.osmocom.org/osmocom/osmo-ci
  $ cd libosmocore
  $ ../osmo-ci/scripts/obs/gerrit_binpkgs.sh debian:12  # or debian:11, debian:10, centos:8

Change-Id: I100d8dfc0c58bdafe7efb0fa4108031ce10398a5