tools: mkeficapsule: Add support to print capsule headers

Add support to dump the contents of capsule headers. This is useful as
a debug feature for checking the contents of the capsule headers, and
can also be used in capsule verification.

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