image: Support signing of images

Add support for signing images using a new signature node. The process
is handled by fdt_add_verification_data() which now takes parameters to
provide the keys and r

image: Support signing of images

Add support for signing images using a new signature node. The process
is handled by fdt_add_verification_data() which now takes parameters to
provide the keys and related information.

Signed-off-by: Simon Glass <sjg@chromium.org>

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