kbuild: drop descend - converting existing users

There was only two users left of descend. Fix them so they
use $(clean)= and $(build)=.
Drop definition of descend.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
---
diff --git a/arch/m68knommu/Makefile b/arch/m68knommu/Makefile
index a254aa9..58c9fa5 100644
--- a/arch/m68knommu/Makefile
+++ b/arch/m68knommu/Makefile
@@ -109,7 +109,7 @@
 prepare: include/asm-$(ARCH)/asm-offsets.h
 
 archclean:
-	$(call descend arch/$(ARCH)/boot, subdirclean)
+	$(Q)$(MAKE) $(clean)=arch/m68knommu/boot
 
 include/asm-$(ARCH)/asm-offsets.h: arch/$(ARCH)/kernel/asm-offsets.s \
 				   include/asm include/linux/version.h \