Use environment variable for download server

To ensure tf-a-ci-scripts can run on both OpenCI
and Arm CI, replace ci.trustedfirmware.org with
enironment variable.

DOWNLOAD_SERVER_URL is a system-wide environment
variable and was configured in:
Manage Jenkins > System > Global Properties

Change-Id: I53bbb8766c9264c7cc1200f8884324b6650f31da
Signed-off-by: Saheer Babu <saheer.babu@arm.com>
6 files changed