Add JOBS_REFSPEC parameter to jobs

Add JOBS_RESPEC parameter to jobs, and set default value
to master branch.

Change-Id: I779f163dc494df07514f4f0b5034c6113afda109
Signed-off-by: Saheer Babu <saheer.babu@arm.com>
(cherry picked from commit 1e3f00ec2b70b98a945e7a4104bb0eca37dcac0b)
diff --git a/tf-a-builder.yaml b/tf-a-builder.yaml
index 0be88a9..d769e7d 100644
--- a/tf-a-builder.yaml
+++ b/tf-a-builder.yaml
@@ -46,6 +46,11 @@
             name: CI_REFSPEC
             default: '+refs/heads/master:refs/remotes/origin/master'
         - string:
+            name: JOBS_REFSPEC
+            default: 'refs/heads/master'
+            description: |
+              tf-a-job-configs refspec to use. The master branch is used by default.
+        - string:
             name: DOCKER_REGISTRY
             default: ${JENKINS_DOCKER_REGISTRY}
             description: JENKINS_DOCKER_REGISTRY is a system-wide environment variable