binman: Allow disabling symbol writing

Some boards don't use symbol writing but do access the symbols in SPL.
Provide an option to work around this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index 3945690..328b5bc 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -158,6 +158,7 @@
         self.offset_from_elf = None
         self.preserve = False
         self.build_done = False
+        self.no_write_symbols = False
 
     @staticmethod
     def FindEntryClass(etype, expanded):
@@ -321,6 +322,7 @@
                                                              'offset-from-elf')
 
         self.preserve = fdt_util.GetBool(self._node, 'preserve')
+        self.no_write_symbols = fdt_util.GetBool(self._node, 'no-write-symbols')
 
     def GetDefaultFilename(self):
         return None
@@ -695,7 +697,7 @@
         Args:
           section: Section containing the entry
         """
-        if self.auto_write_symbols:
+        if self.auto_write_symbols and not self.no_write_symbols:
             # Check if we are writing symbols into an ELF file
             is_elf = self.GetDefaultFilename() == self.elf_fname
             elf.LookupAndWriteSymbols(self.elf_fname, self, section.GetImage(),