tools: add mkeficapsule command for UEFI capsule update

This is a utility mainly for test purpose.
  mkeficapsule -f: create a test capsule file for FIT image firmware

Having said that, you will be able to customize the code to fit
your specific requirements for your platform.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>