doc: update UEFI document for usage of mkeficapsule

Now we can use mkeficapsule command instead of EDK-II's script
to create a signed capsule file. So update the instruction for
capsule authentication.

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