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/arch/mips/config.mk b/arch/mips/config.mk
index c3f81b5..2abdebb 100644
--- a/arch/mips/config.mk
+++ b/arch/mips/config.mk
@@ -50,4 +50,4 @@
 PLATFORM_LDFLAGS		+= -G 0 -static -n -nostdlib $(ENDIANNESS)
 PLATFORM_RELFLAGS		+= -ffunction-sections -fdata-sections
 LDFLAGS_FINAL			+= --gc-sections -pie
-OBJCFLAGS			+= --remove-section=.dynsym
+OBJCOPYFLAGS			+= --remove-section=.dynsym