binman: capsule: Use dumped capsule header contents for verification

The various fields of a generated capsule are currently verified
through hard-coded offsets. Use the dump-capsule feature for dumping
the capsule header contents and use those for capsule verification.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>