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>