mkimage: ecdsa: add signature/key nodes to dtb if missing

mkimage: ecdsa: add signature/key nodes to dtb if missing

If the signature/key node(s) are not yet present in the U-Boot device
tree, ecdsa_add_verify_data simply fails if it can't find the nodes.
This behaviour differs from rsa_add_verify_data, wich does add the missing
nodes and proceeds in that case.

This change is mainly copy&paste from rsa_add_verify_data to add the
same behaviour to ecdsa_add_verify_data.

Signed-off-by: Matthias Pritschet <matthias.pritschet@itk-engineering.de>