The former macro $(descend ...) hide for make the fact that a recursively make was
invoked. The replacement $(Q)$(MAKE) -f scripts/Makefile.build obj=dir was too verbose.
  
Introduced $(build) and $(clean) allowing the following syntax:
$(Q)$(MAKE) $(build)=arch/i386/boot target
and similar for clean.
  
Introduced $(build) and $(clean) in general, and for i386 architecture.