event logger: reduce sample application's RAM footprint

The event logger nanokernel sample application spawns
application fibers and system fibers. This commit reduce
the application's fiber stacks to reduce the global
application RAM footprint and allow it to run on smaller
platforms like Quark D2000.

Jira: ZEP-526

Change-Id: I98f899892e5c5d8ce84f97f6768444cf67277a1a
Signed-off-by: Juan Manuel Cruz <juan.m.cruz.alcaraz@intel.com>