sysbuild: Apply FILE_SUFFIX to sysbuild.conf

Unless `SB_CONF_FILE` is set by the user, an optional `sysbuild.conf`
file is taken by default. If `FILE_SUFFIX=foo` is set, then it should
also be possible to find `sysbuild_foo.conf`, just like with `prj.conf`.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>