merge_config.sh: fix bug in final check

Arnaud Lacombe pointed out the final checking that the requested configs
were included in the final .config was broken.

The example was that if you had a fragment that disabled
CONFIG_DECOMPRESS_GZIP applied to a normal defconfig, there would be no
final warning that CONFIG_DECOMPRESS_GZIP was acutally set in the final
.config.

This bug was introduced by me in v3 of the original patch, and the
following patch reverts the invalid change.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Reported-by: Arnaud Lacombe <lacombar@gmail.com>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>