Sdk: Skip generation of RuntimeConfig.json if it's up to date and there is not runtimeconfig.template.json file

[dsplaisted]

See #471

The target generating RuntimeConfig.json has an Inputs which includes runtimeconfig.template.json, which is often not present, causing the target to always execute. The file should only be included as an input if it exists, to fix incremental compilation.

[rainersigwald]

Including the target name GenerateBuildRuntimeConfigurationFiles in this issue since that's what I searched for when I failed to find this and duplicated it as #944.