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/config.mk b/config.mk
index 5b886aa..9897f20 100644
--- a/config.mk
+++ b/config.mk
@@ -41,7 +41,7 @@
 
 RELFLAGS= $(PLATFORM_RELFLAGS)
 
-OBJCFLAGS += --gap-fill=0xff
+OBJCOPYFLAGS += --gap-fill=0xff
 
 CPPFLAGS = $(RELFLAGS)
 CPPFLAGS += -pipe $(PLATFORM_CPPFLAGS)