binman: Allow support for writing a size symbol to binaries

It is useful to be able to access the size of an image in SPL, with
something like:

binman_sym_declare(unsigned long, u_boot_any, size);

...
   ulong u_boot_size = binman_sym(ulong, u_boot_any, size);

Add support for this and update the tests.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/elf_test.py b/tools/binman/elf_test.py
index f05545b..c0c11cb 100644
--- a/tools/binman/elf_test.py
+++ b/tools/binman/elf_test.py
@@ -155,11 +155,11 @@
         This should produce -1 values for all thress symbols, taking up the
         first 16 bytes of the image.
         """
-        entry = FakeEntry(20)
+        entry = FakeEntry(24)
         section = FakeSection(sym_value=None)
         elf_fname = self.ElfTestFile('u_boot_binman_syms')
         syms = elf.LookupAndWriteSymbols(elf_fname, entry, section)
-        self.assertEqual(tools.GetBytes(255, 16) + tools.GetBytes(ord('a'), 4),
+        self.assertEqual(tools.GetBytes(255, 20) + tools.GetBytes(ord('a'), 4),
                                                                   entry.data)
 
     def testDebug(self):