binman: Move GetFdtSet() into blob_dtb

At present we check the filename to see if an entry holds a device-tree
file. It is easier to use the base class designed for this purpose.

Move this method implementation into Entry_blob_dtb and update the default
one to return an empty set.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/entry.py

@@ -192,11 +192,6 @@ class Entry(object):
             Set containing the filename from this entry, if it is a .dtb, else
             an empty set
         """
-        fname = self.GetDefaultFilename()
-        # It would be better to use isinstance(self, Entry_blob_dtb) here but
-        # we cannot access Entry_blob_dtb
-        if fname and fname.endswith('.dtb'):
-            return set([fname])
         return set()
 
     def ExpandEntries(self):

tools/binman/etype/blob_dtb.py

@@ -31,3 +31,12 @@ class Entry_blob_dtb(Entry_blob):
         _, indata = state.GetFdtContents(self._filename)
         data = self.CompressData(indata)
         return self.ProcessContentsUpdate(data)
+
+    def GetFdtSet(self):
+        """Get the set of device trees used by this entry
+
+        Returns:
+            Set containing the filename from this entry
+        """
+        fname = self.GetDefaultFilename()
+        return set([fname])