kbuild: mergeconfig: move an error check to merge_config.sh

Currently, "make tinyconfig" does not work with "-j" option.

  $ make mrproper
  $ make -j8 tinyconfig
    HOSTCC  scripts/basic/fixdep
    HOSTCC  scripts/kconfig/conf.o
    SHIPPED scripts/kconfig/zconf.tab.c
    SHIPPED scripts/kconfig/zconf.lex.c
    SHIPPED scripts/kconfig/zconf.hash.c
    HOSTCC  scripts/kconfig/zconf.tab.o
    HOSTLD  scripts/kconfig/conf
  scripts/kconfig/conf --allnoconfig Kconfig
  #
  # configuration written to .config
  #
  scripts/kconfig/Makefile:122: *** You need an existing .config
  for this target.  Stop.
  make: *** [tinyconfig] Error 2

As shown above, "allnoconfig" has created the .config file before
mergeconfig is called, but Make still raises a false alarm because
of some sort of race condition.

We can fix this issue by moving the error check to the shell script.

Anyway, scripts/kconfig/merge_config.sh always requires an existing
.config as a base file.  It is reasonable to check its existence in
the shell script.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>