Add support for vpl_common_script.sh

This script, if present, is run very early, before some files are possibily removed

Basic_System/ef/vpl_execution.sh

 #!/bin/bash
+
+if [ -f vpl_common_script.sh ]; then
+    . vpl_common_script.sh
+fi
+
 # Files to keep until the very last moment
 PRECIOUS_FILES="vpl_custom_setup.sh vpl_custom_run.sh"