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
1 file changed