#1268 implement boardDescription.isValid

This is only needed for data migration.
I only check on the existance of the boards.txt file

Author: jantje

io.sloeber.core/src/io/sloeber/core/api/BoardDescription.java

@@ -1128,8 +1128,10 @@ public static BoardDescription getFromCDT(ICConfigurationDescription confDesc) {
     }
 
     public boolean isValid() {
-        // TODO Auto-generated method stub
-        return false;
+        if (myreferencingBoardsFile == null) {
+            return false;
+        }
+        return myreferencingBoardsFile.exists();
     }
 
 }