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>
diff --git a/tools/binman/image.py b/tools/binman/image.py
index 68126bc..1fb5eb6 100644
--- a/tools/binman/image.py
+++ b/tools/binman/image.py
@@ -54,6 +54,10 @@
             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