Fix to prevent loose AXI buses from preventing the beagle from booting.

Fix to prevent loose AXI buses from preventing the beagle from booting. If the argument AXI_STOP_CAP:1 is added to the yaml file, a script is called within the top level script which ties the relevant signals to ground.

As a test of this, the M2_OPTION:AXI_STOP_TEST argument can be passed which generates loose AXI buses.