binman: Obtain the list of device trees from the config

We always have a device tree for U-Boot proper. But we may also have one
for SPL and TPL. Add a new Entry method to find out what DTs an entry
has, and use that list when updating DTs.

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

tools/binman/image.py

@@ -54,6 +54,10 @@ class Image:
             self._filename = filename
         self._section = bsection.Section('main-section', self._node)
 
+    def GetFdtSet(self):
+        """Get the set of device tree files used by this image"""
+        return self._section.GetFdtSet()
+
     def AddMissingProperties(self):
         """Add properties that are not present in the device tree