Core: Fix NULL thread indicator caused fault

The current thread indicator 'CURR_THRD' is NULL before first
context switching. This leads to save runtime context into a NULL
address and cause a Bus Fault in the specified platform. Create a
dedicated scheduler start function to set a CURR_THRD default value
and add assert to ensure the integrity of thread pointers.

Change-Id: Ifcd3f10c8e648adccadecc2ee3ea70a9c318d7cf
Signed-off-by: Ken Liu <ken.liu@arm.com>