kbuild: rename OBJCFLAGS to OBJCOPYFLAGS

Rename OBJCFLAGS to OBJCOPYFLAGS beforehand to use
"cmd_objcopy" in scripts/Makefile.lib in an upcoming commit.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/spl/Makefile b/spl/Makefile
index 9d3d182..b983039 100644
--- a/spl/Makefile
+++ b/spl/Makefile
@@ -201,7 +201,7 @@
 endif
 
 $(obj)/$(SPL_BIN).bin:	$(obj)/$(SPL_BIN)
-	$(OBJCOPY) $(OBJCFLAGS) $(SPL_OBJCFLAGS) -O binary $< $@
+	$(OBJCOPY) $(OBJCOPYFLAGS) $(SPL_OBJCFLAGS) -O binary $< $@
 
 LDFLAGS_$(SPL_BIN) += -T u-boot-spl.lds $(LDFLAGS_FINAL)
 ifneq ($(CONFIG_SPL_TEXT_BASE),)