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/mips/Makefile b/arch/mips/Makefile
index bc1c442..26528b6 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -683,7 +683,7 @@
 
 ifdef CONFIG_LASAT
 rom.bin rom.sw: vmlinux
-	$(call descend,arch/mips/lasat/image,$@)
+	$(Q)$(MAKE) $(build)=arch/mips/lasat/image $@
 endif
 
 #