Correct U-Boot upstream repository

The U-Boot source moves to https://source.denx.de/u-boot/u-boot.git
effective 2021-02-28.

Signed-off-by: Heinrich Schuchardt <[email protected]>

Author: xypron

.gitlab-ci.yml

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0+
 
 # Grab our configured image.  The source for this is found at:
-# https://gitlab.denx.de/u-boot/gitlab-ci-runner
+# https://source.denx.de/u-boot/gitlab-ci-runner
 image: trini/u-boot-gitlab-ci-runner:bionic-20200807-02Sep2020
 
 # We run some tests in different order, to catch some failures quicker.