xenclient-oe.git : Clone openxt build metadata with all the other meta-layers.

We use the same mechanism to clone xenclient-oe.git now as well. No
need to clone it directly in do_build.sh any longer as the local.conf
isn't in there any longer. We can use the same mechanism in
setup_build.

Signed-off-by: Philip Tricca <flihp@twobit.us>