binman: Handle reading data for end-at-4gb sections

Some x86 sections have special offsets which currently result in empty
data being returned from the 'extract' command. Fix this by taking account
of the skip-at-start property.

Add a little more debugging while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Bin Meng <bmeng.cn@gmail.com>

tools/binman/image.py

@@ -201,6 +201,8 @@ class Image(section.Entry_section):
         return entry
 
     def ReadData(self, decomp=True):
+        tout.Debug("Image '%s' ReadData(), size=%#x" %
+                   (self.GetPath(), len(self._data)))
         return self._data
 
     def GetListEntries(self, entry_paths):