kernel.core: change logger to tracer.

kernel.core: change logger to tracer.

The logger generates a large amount of repeated data, this should go in
a trace. Additional data is also stored, but it is faster if tracing
is disabled as the string is not generated.

This should reduce the build log file sizes noticeably.

Change-Id: I7e1c2a799b9386be5872fe7ac3a3e4686ae416c7
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/98173
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI