binman: Assume Intel descriptor is at the start of the image

At present binman requires that the Intel descriptor has an explicit
offset. Generally this is 0 since the descriptor is at the start of the
image. Add a default to handle this, so users don't need to specify the
offset.

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

tools/binman/bsection.py

@@ -477,6 +477,14 @@ class Section(object):
             return self._parent_section.GetRootSkipAtStart()
         return self._skip_at_start
 
+    def GetStartOffset(self):
+        """Get the start offset for this section
+
+        Returns:
+            The first available offset in this section (typically 0)
+        """
+        return self._skip_at_start
+
     def GetImageSize(self):
         """Get the size of the image containing this section
 

tools/binman/etype/intel_descriptor.py

@@ -47,6 +47,8 @@ class Entry_intel_descriptor(Entry_blob):
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
         self._regions = []
+        if self.offset is None:
+            self.offset = self.section.GetStartOffset()
 
     def GetOffsets(self):
         offset = self.data.find(FD_SIGNATURE)