kbuild: update config_data.gz only when the content of .config is changed

If the timestamp of the .config file is updated, config_data.gz is
regenerated, then vmlinux is re-linked. This occurs even if the content
of the .config has not changed at all.

This issue was mitigated by commit 67424f61

 ("kconfig: do not write
.config if the content is the same"); Kconfig does not update the
.config when it ends up with the identical configuration.

The issue is remaining when the .config is created by *_defconfig with
some config fragment(s) applied on top.

This is typical for powerpc and mips, where several *_defconfig targets
are constructed by using merge_config.sh.

One workaround is to have the copy of the .config. The filechk rule
updates the copy, kernel/config_data, by checking the content instead
of the timestamp.

With this commit, the second run with the same configuration avoids
the needless rebuilds.

  $ make ARCH=mips defconfig all
   [ snip ]
  $ make ARCH=mips defconfig all
  *** Default configuration is based on target '32r2el_defconfig'
  Using ./arch/mips/configs/generic_defconfig as base
  Merging arch/mips/configs/generic/32r2.config
  Merging arch/mips/configs/generic/el.config
  Merging ./arch/mips/configs/generic/board-boston.config
  Merging ./arch/mips/configs/generic/board-ni169445.config
  Merging ./arch/mips/configs/generic/board-ocelot.config
  Merging ./arch/mips/configs/generic/board-ranchu.config
  Merging ./arch/mips/configs/generic/board-sead-3.config
  Merging ./arch/mips/configs/generic/board-xilfpga.config
  #
  # configuration written to .config
  #
    SYNC    include/config/auto.conf
    CALL    scripts/checksyscalls.sh
    CALL    scripts/atomic/check-atomics.sh
    CHK     include/generated/compile.h
    CHK     include/generated/autoksyms.h

Reported-by: Elliot Berman <eberman@codeaurora.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>