layout.conf: set repo-name

We want to drop repo_name support, so move it to layout.conf.

BUG=chromium:386495
TEST=`cbuildbot chromiumos-sdk` passes
TEST=pre-cq passes; tree doesn't burn down

Change-Id: I63bacc619b947fe218c92bb10c265fd5adaa380a
Reviewed-on: https://chromium-review.googlesource.com/227263
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>