soc: xtensa: split the cavs server and the FW loader

Spilt the cavs server and FW loader, to make the cavstool.py
a pure FW loader and runner as it used to be. The reasons is
try to keep the FW loader won't be affected by the client-
server-based HW service as possible, and more easy to debug.

After splitting them, the service program spawns another
process to run the FW loader. The advantage is no matter when
the FW loader is stuck, the service can detect and stop it,
then continue to next test.

Fixes #47652.

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