target-xtensa: add test_kc705_be core

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: add dc233_xea3 core

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: add core importing script

This script copies configuration and gdb information from the xtensa
configuration overlay archive and registers new xtensa core.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>