mkimage: Allow to specify the signature algorithm on the command line

This permits to prepare FIT image description that do not hard-code the
final choice of the signature algorithm, possibly requiring the user to
patch the sources.

When -o <algo> is specified, this information is used in favor of the
'algo' property in the signature node. Furthermore, that property is set
accordingly when writing the image.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>