boards: xtensa: Activate the intel_adsp west runner

Make the intel_adsp west runner starting to work on all the
intel_adsp boards. Changes include:

1. Make the cavstool.py work as a service in remote host
   ADSP board and rename it to cavstool_server.py.

2. Active the runner and adds a common board.cmake file to
   specify the default signing key for cavs boards.

Signed-off-by: Enjia Mai <enjia.mai@intel.com>