x86: Reduce resetvec size

At present this produces a 16-byte file. It is intended to start 16 bytes
before the end of ROM and pads with zeroes to readh the end.

But binman sometimes wants to add an image-header at the very end of ROM.
Furthermore binman automatically pads the data if it is finishes early.

Drop the padding in resetvec and let binman handle it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>