build: Add clean file variables

This replaces the k_clean and clean targets with a system of CLEAN_FILES
variables which can be used to mark files for removal.
build: Don't print error output from find
build: Add a k_sources variable

This combines the $(k_c_sources) and $(k_asm_sources) variables.
build: Add cscope database support
build: Add $(MRPROPER_FILES) values
build: Add $(DISTCLEAN_FILES) values