isst-config.c should only contain generic code.
Move mbox functions which are platform specific code to isst-core.c.

As there are some platform specific parameters set via generic
application options, introduce isst_update_platform_param to pass these
parameters to platform specific code.
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>