kbuild: fix dependency of DT build

kbuild: fix dependency of DT build

I saw a DT build issue report some time before [1].  I was able to
reproduce the bug, and figure out the root cause.

Since commit 6d427c6b1fa0 ("binman: Automatically include a U-Boot
.dtsi file"), invalid .*.cmd files are generated.

Since that commit, DTS files are put into sed and piped to CPP.
Because CPP reads the stream from stdin, -Wp,-MD,$(depfile).pre.tmp
option generates a depfile with the target name "-".  This is not
the format expected by fixdep.

Use one more temporary file instead of using pipe.  With this, deps_
in the .*.cmd will be fixed.  Having a temp file name in source_ is
odd, but it is unsed in the build system.  Not a big deal.

[1] https://lists.denx.de/pipermail/u-boot/2017-June/294451.html

Fiexes: 6d427c6b1fa0 ("binman: Automatically include a U-Boot .dtsi file")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>