repo: Add check of REPO_URL env variable

We want to be able to run repo on a system that is not connected to
the Internet and cannot access https://gerrit.googlesource.com. We
can put a clone of that repos there, but would prefer to use the
stable version of the repo script instead of a locally modified
version.

This commit adds a check for the REPO_URL environment variable. If
that is set and not empty its value will be set in the REPO_URL
global in repo.  Otherwise the standard path will be used.

Change-Id: I0616f5f81ef75f3463b73623b892cb5eed6bb7ba

repo

@@ -2,7 +2,10 @@
 
 ## repo default configuration
 ##
-REPO_URL = 'https://gerrit.googlesource.com/git-repo'
+import os
+REPO_URL = os.environ.get('REPO_URL', None)
+if not REPO_URL:
+  REPO_URL = 'https://gerrit.googlesource.com/git-repo'
 REPO_REV = 'stable'
 
 # Copyright (C) 2008 Google Inc.
@@ -114,7 +117,6 @@ GITC_FS_ROOT_DIR = '/gitc/manifest-rw/'
 
 import errno
 import optparse
-import os
 import re
 import shutil
 import stat