microkernel: introduce private event objects

This patch enables defining microkernel events within source code.
This is similar to other private kernel object patches.

The test has been modified a little bit due to the fact that
the event ID is now a memory address, instead of numeric ID.

Change-Id: Ie3c8d4f4e459d9c631e50bb242cf7a05ca8ea82c
Signed-off-by: Daniel Leung <daniel.leung@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>