sysbuild: fix extra kconfig targets when using sysbuild

Zephyr provides a feature to hook in custom menuconfig implementations
by setting EXTRA_KCONFIG_TARGETS and then defining the corresponding
command to invoke as EXTRA_KCONFIG_TARGET_COMMAND_FOR_<target>.

This feature is broken with sysbuild because sysbuild will try to create
multiple custom targets with identical names.

This commit fix the sysbuild handling of extra kconfig targets.

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