Move the loopback test to the greybus main repo, as we will be adding
more tests over time and it doesn't need to be burried in the gbsim
repo.

This moves the latest version from gbsim to this repo and fixes up the
Makefile to be a bit more "smart" when building the code.
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>