Currently the run_cmd, which is a wrapper for self.target.run()
that uses SSH to spawn commands on the target, can fail spuriously
with error 255 and cause the test to fail on slow systems.

In order to address that, introduce a retry mechanism for the call,
that is able to wait some time for the system to settle and retry
the command when the error code from SSH is 255.
Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>