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: NJohn Stultz <john.stultz@linaro.org>
Acked-by: NDarren Hart <dvhart@linux.intel.com>
Acked-by: NBruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: NMichal Marek <mmarek@suse.cz>