[OXT-24] Treat the xenclient-oe.git repo like it was any other meta layer.

do_build.sh & setup_build: Clone xenclient-oe.git using the same
  mechanism we use for the other meta layers and we clone it in the
  same location.
local.conf-dist: Move yet another build file from xenclient-oe.git
  to the build scripts repo.