sysbuild: support sample configuration of extra images

This commit introduces the possibility of a sample to locate
configuration files for extra images that are used when building with
MCUboot.

This allows use-cases where a sample, A, want to include MCUboot but has
adjustments to the default MCUboot configuration.

By adding a Kconfig fragment `<sample>/sysbuild/mcuboot.conf`, then that
fragment will be used together with the default configuration for
MCUboot.

It is also possible to completely replace the MCUboot configuration.
This is done by creating `<sample>/sysbuild/mcuboot/` folder.
This folder will then be used as the `APPLICATION_CONFIG_DIR` when
building MCUboot.

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