Using the recent GNU ARM Eclipse v2.11 
(https://gnuarmeclipse.github.io/) 
and following the installation instructions to the letter
including the Packs Download (takes a while).
The eclipse-mars-gnuarm-mchf-preferences.epf contains the exported 
preferences from Eclipse Mars with a working debug configuration.
If you already configured Eclipse, you may not want to use it, then
follow the "images" in Eclipse-Debug-Settings to create a debug 
configuration by making the  entries marked with red boxes in 
a new GDB OpenOCD Debug Configuration. 
You should be able to debug live with an ST-LINK V2 in 
no time at  all.