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>
diff --git a/script/tf-coverity/tf-cov-make b/script/tf-coverity/tf-cov-make
index 14ccf63..462e15e 100755
--- a/script/tf-coverity/tf-cov-make
+++ b/script/tf-coverity/tf-cov-make
@@ -544,7 +544,7 @@
 
 # Source files from mv-ddr-marvell repository are necessary
 # to build below four platforms
-wget https://downloads.trustedfirmware.org/tf-a/mv-ddr-marvell/mv-ddr-marvell-5d41a995637de1dbc93f193db6ef0c8954cab316.tar.gz 2> /dev/null
+wget ${DOWNLOAD_SERVER_URL}/tf-a/mv-ddr-marvell/mv-ddr-marvell-5d41a995637de1dbc93f193db6ef0c8954cab316.tar.gz 2> /dev/null
 tar -xzf mv-ddr-marvell-5d41a995637de1dbc93f193db6ef0c8954cab316.tar.gz 2> /dev/null
 mv mv-ddr-marvell drivers/marvell/mv_ddr