west: core: allow to pass keywords args to server and client

Allow the runners to pass extra keywords arguments to both the server
and client subprocesses calls, as can be useful for instance to set
a specific execution environment.
ZephyrBinaryRunner.check_call() already supports passing kwargs to
the inner subprocess call but this is currently not possible when
starting server/client processes. So pass through the kwargs to each of
the client/server subprocess calls.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>