FreeRTOS: Make optional symbols optional

FreeRTOS: Make optional symbols optional

xSuspendedTaskList and xTasksWaitingTermination are only available for
some configurations. Missing optional symbols will have their addresses
remaining at zero so the corresponding lists will be skipped when
building the task list.

Change-Id: If330f5038d009298c3a14a4d2756db7105a30bc8
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2425
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>