The tftpboot build was failing with missing file errors.

It turns out that $(obj)/image wasn't being generated which was causing the a.out conversion to be skipped and hence piggyback to be called with nonexistent files.
Signed-off-by: NJulian Calaby <julian.calaby@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>