Cleanup kernels in target which don't have a source

[bryteise]

Make another cleanup step in clr-boot-manager which will look at all the names in the target and try to find those files' matching sources. If the matching sources of those files do not exist and the kernel isn't being used (either as default or as a fallback), then cleanup all the target files.

[dorileo]

Trying to figure the actual use case.