Introduce sysbuild-specific variants of existing Kconfig checks:
* SysbuildKconfig
* SysbuildKconfigBasic
* SysbuildKconfigBasicNoModules
This involves a few additions to the base `KconfigCheck` class:
* Supporting a variable symbol prefix, to handle `SB_CONFIG_`.
* Generating extra files, including `Kconfig.sysbuild.modules`.
Although these are never sourced outside of sysbuild Kconfig,
they're still generated for every regular Zephyr build, so it's
natural to let all Kconfig checks follow this behavior.
Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>
