binman: Ensure attributes always come last in the metadata

cbfsutil changed to write zero bytes instead of 0xff when a small
padding must be added. Adjust the binman implementation to do the same.

Drop the code which looks for an unused attribute tag, since it is not
used. A future patch moves the attributes to the end of the header in
any case, so no data will follow the attributes.

This mirrors commit f0cc7adb2f in coreboot.

Signed-off-by: Simon Glass <sjg@chromium.org>