static-checks: use origin/master instead of master as base branch

In some CI testing scenarios, there are no local branches, just remotes, so
as a safer approach, use 'origin/master' instead of 'master'. Base
branch is used to find common parent ancestor between
base (origin/master) and user patchset's (FETCH_HEAD) branches.

Change-Id: I36940ac043f45bda6b94e8787b29f319a5f4acba
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
diff --git a/script/static-checks/check-copyright.py b/script/static-checks/check-copyright.py
index 39863c7..d99aa5d 100755
--- a/script/static-checks/check-copyright.py
+++ b/script/static-checks/check-copyright.py
@@ -180,7 +180,7 @@
 only files that are modified by the latest patch(es).""",
                         action="store_true")
 
-    (rc, stdout, stderr) = utils.shell_command(['git', 'merge-base', 'HEAD', 'master'])
+    (rc, stdout, stderr) = utils.shell_command(['git', 'merge-base', 'HEAD', 'origin/master'])
     if rc:
         print("Git merge-base command failed. Cannot determine base commit.")
         sys.exit(rc)