binman: Assume Intel descriptor is at the start of the image

binman: Assume Intel descriptor is at the start of the image

At present binman requires that the Intel descriptor has an explicit
offset. Generally this is 0 since the descriptor is at the start of the
image. Add a default to handle this, so users don't need to specify the
offset.

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