Make so versioning forward compatibile

Make so versioning forward compatibile

The commit 317ef5442d048a5b0701748aaae1fd92e6169d97 modified the so
versioning while keeping backward compatibility.

It turns out that we also have to keep it forward compatible as apps
built with latest dcm may be installed on older images. Currently it's
not possible as the lib*.so.0 used by newly built apps is absent on old
images.

This commit sets the SO VERSION to 2.0 to provide forward compatibility.

Old dcm:
- main rpm: lib*.so.2.0
- devel rpm: lib*.so -> lib*.so.2.0

Backward compatible dcm:
- main rpm:
  - lib*.so.2.1.2
  - lib*.so.0 -> lib*.so.2.1.2
  - lib*.so.2.0 -> lib*.so.2.1.2 (link added manually in spec)
- devel rpm: lib*.so -> lib*.so.0

Forward & backward compatible dcm (this commit):
- main rpm:
  - lib*.so.2.1.2
  - lib*.so.2.0 -> lib*.so.2.1.2
- devel rpm: lib*.so -> lib*.so.2.0

Change-Id: I1bec54fc62eecec56e54f64e3acd6bb1b7217336