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/board/hymod/config.mk b/board/hymod/config.mk
index abcd2d5..ee15890 100644
--- a/board/hymod/config.mk
+++ b/board/hymod/config.mk
@@ -11,4 +11,4 @@
 
 PLATFORM_CPPFLAGS += -I$(TOPDIR)
 
-OBJCFLAGS = --remove-section=.ppcenv
+OBJCOPYFLAGS = --remove-section=.ppcenv