Add definition of images which elxr CI pipeline uses.

eLxr provides a Linux distribution for users that want a solution that gets them up and running fast, is easy to modify, and works with on-premise and edge (cloud-based) solutions; all while being compatible with popular development and integration tools. The eLxr project builds on Debian’s stability, reliability, and commercial-off-the-shelf (COTS) hardware support for hardening Debian for near edge server requirements. The result is a guaranteed, freely accessible open source community–innovated Debian ready for edge-to-cloud workloads.

eLxr repo is at https://gitlab.com/elxr.

eLxr builds packages using salsa pipeline.

The commit adds definition of images which eLxr CI pipeline uses. It enables eLxr image build job with variable 'BUILD_ELXR_IMAGES'.