cmake: sysbuild: signing support

This commit introduces image signing by adding the possibility to
specify algorithm and signing key for sysbuild images.

It introduces Kconfig setting to specify signing algorithm and key file.

It will default the signing key to the default key provided by MCUBoot
if no key has been specified.

When signing is enabling, the signature key will be passed to the
application so the build system can sign the image as post build step.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>