tmf.xml: Add support for peek() stack operation

tmf.xml: Add support for peek() stack operation

This implements the peek operation for a stack of state value. On a
condition, the peek operation read the value at the top without
removing. On a state change, the peek operation will do nothing. A test
for a stack state value is provided.

Change-Id: I2192ee41171a7c6eabce2941466edbc168c1fa82
Signed-off-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/77274
Reviewed-by: Hudson CI
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>