lttng: Update README for building of LTTng p2 update site

lttng: Update README for building of LTTng p2 update site

Additionally, add state system plug-ins in list of plug-ins and
change default path name of lttngPackageDestination.

Change-Id: Ia69d532d86861c4246c3808355453d2f227e4b04
Signed-off-by: Bernd Hufmann <Bernd.Hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/29029
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>