MIPS: boot: fix build rule of vmlinux.its.S

As Documentation/kbuild/makefile.txt says, it is a typical mistake
to forget the FORCE prerequisite for the rule invoked by if_changed.

Add the FORCE to the prerequisite, but it must be filtered-out from
the files passed to the 'cat' command.  Because this rule generates
.vmlinux.its.S.cmd, vmlinux.its.S must be specified as targets so
that the .cmd file is included.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Patchwork: https://patchwork.linux-mips.org/patch/19097/Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org

arch/mips/boot/Makefile

@@ -118,10 +118,12 @@ ifeq ($(ADDR_BITS),64)
 	itb_addr_cells = 2
 endif
 
+targets += vmlinux.its.S
+
 quiet_cmd_its_cat = CAT     $@
-      cmd_its_cat = cat $^ >$@
+      cmd_its_cat = cat $(filter-out $(PHONY), $^) >$@
 
-$(obj)/vmlinux.its.S: $(addprefix $(srctree)/arch/mips/$(PLATFORM)/,$(ITS_INPUTS))
+$(obj)/vmlinux.its.S: $(addprefix $(srctree)/arch/mips/$(PLATFORM)/,$(ITS_INPUTS)) FORCE
 	$(call if_changed,its_cat)
 
 quiet_cmd_cpp_its_S = ITS     $@