Align DOCKER_PREFIX with PRIVATE_CONTAINER_REGISTRY

PRIVATE_CONTAINER_REGISTRY is a system wide variable
set in jenkins. This is already used by tf-a jobs.
Rather than creating another variable align this in
job config. Renaming DOCKER_PREFIX to PRIVATE_CONTAINER_REGISTRY
will require additional work as it is used in Docker
image and may break developer workflow.

Change-Id: Iae983fa12bb3f0baa81dfd43494cfcfbe819361d
Signed-off-by: Saheer Babu <saheer.babu@arm.com>
diff --git a/tf-m-lava-submit.yaml b/tf-m-lava-submit.yaml
index 5ddf9d7..9423954 100644
--- a/tf-m-lava-submit.yaml
+++ b/tf-m-lava-submit.yaml
@@ -22,6 +22,10 @@
 # e.g. DOCKER_PREFIX, LICENSE_VARIABLE
     parameters:
       - string:
+          name: DOCKER_PREFIX
+          default: ${PRIVATE_CONTAINER_REGISTRY}
+          description: PRIVATE_CONTAINER_REGISTRY is a system-wide environment variable
+      - string:
           name: 'TARGET_PLATFORM'
           description: |
             Specifies the target platform.