tmf : Add comparison between two state values in the XML core

tmf : Add comparison between two state values in the XML core

Previously, it was only possible to compare a state value to a constant.
Now, we can compare directly two state values. With that, it is possible
to query and compare two state values already stored in the state
system. For now, only the "equal" operator is supported.

Change-Id: Iebbcb90c2c0603c5ec1191530e25dd3b0a1cf853
Signed-off-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/61671
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>