Specify location of 'repo' tool during 'repo init'

This is to prevent 'repo' from failing when kernel.org is down.

TEST=Ran trybot on empty buildroot and with an existing buildroot that
requires re-initialization.

Integration of changes http://gerrit.chromium.org/gerrit/#change,7111
and http://gerrit.chromium.org/gerrit/#change,7120 from TOT.

Change-Id: Iae203575d9cf377cc72a87aa5998a5f707b52f26
Reviewed-on: http://gerrit.chromium.org/gerrit/7213
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>
3 files changed