feat: enable spec of upstream_branch
Add support for upstream_branch
configuration; allows "downstream" (Yocto) builds to use specific branches of "upstream" (FPGA) builds
upstream_branch
can point to the branch of upstream build to use; if it's omitted it defaults to the branch name of the current downstream build
Example usage: https://gitlab.desy.de/fpgafw/projects/petra/petra_motion_control/-/blob/jenkins-sdcard/Jenkinsfile.yocto?ref_type=heads