binman: Allow mkimage to use a non-zero fake-blob size

Unfortunately mkimage gets upset with zero-sized files. Update the
ObtainContents() method to support specifying the size, if a fake blob is
created.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>