cmd: add "efidebug capsule" command

"efidebug capsule" is more or less a debugging utility.
  efidebug capsule update: invoke UpdateCapsule against data on memory
  efidebug capsule show: show a capsule header
  efidebug capsule result: dump a capsule result variable

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