binman: Add an image header

It is useful to be able to quickly locate the FDT map in the image. An
easy way to do this is with a pointer at the start or end of the image.

Add an 'image header' entry, which places a magic number followed by a
pointer to the FDT map. This can be located at the start or end of the
image, or at a chosen location.

As part of this, update GetSiblingImagePos() to detect missing siblings.

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

tools/binman/entry.py

@@ -561,3 +561,14 @@ features to produce new behaviours.
                 else False
         """
         return name in self.section.GetEntries()
+
+    def GetSiblingImagePos(self, name):
+        """Return the image position of the given sibling
+
+        Returns:
+            Image position of sibling, or None if the sibling has no position,
+                or False if there is no such sibling
+        """
+        if not self.HasSibling(name):
+            return False
+        return self.section.GetEntries()[name].image_pos