Today there is a huge list of files in the top-level Makefile that is
deleted during make clean and make mrproper.
This patch add infrastructure to get rid of this centralised list.

Within a makefile simply use:
clean-files := files-to-be-deleted
or eventually
clean-rule := command to be executed to delete files
  
Files specified by host-progs and EXTRA_TARGETS are deleted during cleaning,
and the same is all *.[oas] .*.cmd .*.tmp .*.d in the visited directories.
  
Deleting core files is moved down to mrporper time

Patches utilising this and the centralised list will dismiss.

Based on a concept originally made by Kai Germaschewski