merge_config.sh: Set execute bit

Somehow the merge_config.sh script didn't get its execute bit
set when it was merged. Fix this.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Acked-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>