btool: mkeficapsule: Add support for EFI empty capsule generation

Add a method to the mkeficapsule bintool to generate empty
capsules. These are capsules needed for the FWU A/B update feature.

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