Revert "Revert "mkeficapsule: Remove dtb related options""

This reverts commit d428e812.
We have agreed with removing dtb-related stuff from mkeficapsule
command even if the commit 47a25e81 ("Revert "efi_capsule: Move
signature from DTB to .rodata"") was applied.

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