Add support to build and run on Corstone-310

This is controlled by the option "-t" or "--target" that can be passed
to ats commands provided by ats.sh.
The value of the options can be Corstone-300 or Corstone-310
conftest.py has also been updated to support multiple AVH.
Use the option --avh followed by the path to the avh to use to select
an avh.

Signed-off-by: Vincent Coubard <vincent.coubard@arm.com>