Rules.make now has three targets:
o default (a.k.a first_rule): The actual build. Deciding whether
  to build built-in or modular or both is decided by
  $(KBUILD_MODULES) and $(KBUILD_BUILTIN) now, instead of using
  different targets
o fastdep: doesn't actually dependencies anymore, only generates
  modversions
o modules_install: Well, you guess what that does.

Cleaned up descending, and no more differentiating between 
$(subdir-y) and $(subdir-m). That means $(mod-subdirs) can
go away now.