kbuild: deb-pkg: fix bindeb-pkg breakage when O= is used

kbuild: deb-pkg: fix bindeb-pkg breakage when O= is used

Ard Biesheuvel reports bindeb-pkg with O= option is broken in the
following way:

  ...
    LD [M]  sound/soc/rockchip/snd-soc-rk3399-gru-sound.ko
    LD [M]  sound/soc/rockchip/snd-soc-rockchip-pcm.ko
    LD [M]  sound/soc/rockchip/snd-soc-rockchip-rt5645.ko
    LD [M]  sound/soc/rockchip/snd-soc-rockchip-spdif.ko
    LD [M]  sound/soc/sh/rcar/snd-soc-rcar.ko
   fakeroot -u debian/rules binary
  make KERNELRELEASE=4.19.0-12677-g19beffaf7a99-dirty ARCH=arm64 KBUILD_SRC= intdeb-pkg
  /bin/bash /home/ard/linux/scripts/package/builddeb
  Makefile:600: include/config/auto.conf: No such file or directory
  ***
  *** Configuration file ".config" not found!
  ***
  *** Please run some configurator (e.g. "make oldconfig" or
  *** "make menuconfig" or "make xconfig").
  ***
  make[12]: *** [syncconfig] Error 1
  make[11]: *** [syncconfig] Error 2
  make[10]: *** [include/config/auto.conf] Error 2
  make[9]: *** [__sub-make] Error 2
  ...

Prior to commit 80463f1b7bf9 ("kbuild: add --include-dir flag only
for out-of-tree build"), both srctree and objtree were added to
--include-dir redundantly, and the wrong code '$MAKE image_name'
was working by relying on that. Now, the potential issue that had
previously been hidden just showed up.

'$MAKE image_name' recurses to the generated $(objtree)/Makefile and
ends up with running in srctree, which is incorrect. It should be
invoked with '-f $srctree/Makefile' (or KBUILD_SRC=) to be executed
in objtree.

Fixes: 80463f1b7bf9 ("kbuild: add --include-dir flag only for out-of-tree build")
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>