SPM: Enable passing a parameter on thread start

This patch enables callers of thrd_start() to provide a parameter that
will be passed to the thread entry function.
Previously this parameter was hard-coded to be NULL.
This is needed for further SPM modifications.

Change-Id: I5e0d5278867d26bc79b39a8594c3d6ef0eb1e561
Signed-off-by: Chris Brand <chris.brand@cypress.com>
Signed-off-by: Bohdan Hunko <Bohdan.Hunko@infineon.com>