mkeficapsule: Add support for generating empty capsules

mkeficapsule: Add support for generating empty capsules

The Dependable Boot specification[1] describes the structure of the
firmware accept and revert capsules. These are empty capsules which
are used for signalling the acceptance or rejection of the updated
firmware by the OS. Add support for generating these empty capsules.

[1] - https://git.codelinaro.org/linaro/dependable-boot/mbfw/uploads/6f7ddfe3be24e18d4319e108a758d02e/mbfw.pdf

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>