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

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

BugLink: https://bugs.launchpad.net/bugs/1836287
[ Upstream commit 67e09db507db3e1642ddce512a4313d20addd6e5 ]

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
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>