binman: Rename Entry.end_4gb

The property is named end_at_4gb so name the variable the same, to avoid
confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index 68f8d62..e506b55 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -722,7 +722,7 @@
             is_elf = self.GetDefaultFilename() == self.elf_fname
 
             symbols_base = self.symbols_base
-            if symbols_base is None and self.GetImage()._end_4gb:
+            if symbols_base is None and self.GetImage()._end_at_4gb:
                 symbols_base = 0
 
             elf.LookupAndWriteSymbols(self.elf_fname, self, section.GetImage(),