kernel_event_logger: Ignore events before subsystem init
kernel_event_logger: Ignore events before subsystem init

Thread and interrupt events may well arrive before the subsystem
initialization call has been made.  Just swallow such events.

In particular, an incoming change to the way _ready_thread works
causes the main and idle thread initialization to throw thread ready
events (which isn't wrong!), which the current setup can't handle.

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

GiulianoFranchetto/zephyr,ldts/zephyr,kraj/zephyr,punitvara/zephyr,Vudentz/zephyr,ldts/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,zephyriot/zephyr,mbolivar/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,punitvara/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,explora26/zephyr,zephyriot/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,ldts/zephyr,kraj/zephyr,punitvara/zephyr,zephyriot/zephyr,kraj/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,kraj/zephyr,galak/zephyr,mbolivar/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,zephyriot/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,punitvara/zephyr,finikorg/zephyr,galak/zephyr