sunxi: kbuild: convert OBJTREE to objtree

Commit 5ee828c (kbuild: rename OBJTREE to objtree) did what it
said on the tin. Fix sunxi to do the same thing.

How exciting!!!

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
diff --git a/spl/Makefile b/spl/Makefile
index ff3da8d..e7c7a95 100644
--- a/spl/Makefile
+++ b/spl/Makefile
@@ -223,7 +223,7 @@
 
 ifdef CONFIG_SUNXI
 quiet_cmd_mksunxiboot = MKSUNXI $@
-cmd_mksunxiboot = $(OBJTREE)/tools/mksunxiboot $< $@
+cmd_mksunxiboot = $(objtree)/tools/mksunxiboot $< $@
 $(obj)/sunxi-spl.bin: $(obj)/$(SPL_BIN).bin
 	$(call if_changed,mksunxiboot)
 endif