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/test/u_boot_binman_syms.c b/tools/binman/test/u_boot_binman_syms.c
index 4898f98..4520b31 100644
--- a/tools/binman/test/u_boot_binman_syms.c
+++ b/tools/binman/test/u_boot_binman_syms.c
@@ -11,3 +11,4 @@
 binman_sym_declare(unsigned long, u_boot_spl, offset);
 binman_sym_declare(unsigned long long, u_boot_spl2, offset);
 binman_sym_declare(unsigned long, u_boot_any, image_pos);
+binman_sym_declare(unsigned long, u_boot_any, size);