tools: mkeficapsule: add firmware image signing

With this enhancement, mkeficapsule will be able to sign a capsule
file when it is created. A signature added will be used later
in the verification at FMP's SetImage() call.

To do that, we need specify additional command parameters:
  -monotonic-cout <count> : monotonic count
  -private-key <private key file> : private key file
  -certificate <certificate file> : certificate file
Only when all of those parameters are given, a signature will be added
to a capsule file.

Users are expected to maintain and increment the monotonic count at
every time of the update for each firmware image.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>