Add signed FIT image for SabreLite for HAB verification

Allow to prepare single signed FIT image suitable for HAB verification.

Test Jenkins job: https://jenkins.apertis.org/user/d4s/my-views/view/all/job/playground/job/d4s/job/apertis-docker-images/27/

Tested minimal image with signed kernel: https://images.apertis.org/test/playground/d4s/apertis-docker-images/daily/v2021dev1/20200212.16/armhf/minimal/

Tested U-Boot intaller as well.